Chapter 1 General Information

The MySQL™ software delivers a very fast, multithreaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL Server is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. MySQL is a trademark of Oracle Corporation and/or its affiliates, and shall not be used by Customer without Oracle's express written authorization. Other names may be trademarks of their respective owners.

The MySQL software is Dual Licensed. Users can choose to use the MySQL software as an Open Source product under the terms of the GNU General Public License ( or can purchase a standard commercial license from Oracle. See for more information on our licensing policies.

The following list describes some sections of particular interest in this manual:


To report problems or bugs, please use the instructions at Section 1.6, “How to Report Bugs or Problems”. If you find a security bug in MySQL Server, please let us know immediately by sending an email message to . Exception: Support customers should report all problems, including security bugs, to Oracle Support.

1.1 About This Manual

This is the Reference Manual for the MySQL Database System, version 8.0, through release 8.0.24. Differences between minor versions of MySQL 8.0 are noted in the present text with reference to release numbers (8.0.x). For license information, see the Legal Notices.

This manual is not intended for use with older versions of the MySQL software due to the many functional and other differences between MySQL 8.0 and previous versions. If you are using an earlier release of the MySQL software, please refer to the appropriate manual. For example, MySQL 5.7 Reference Manual covers the 5.7 series of MySQL software releases.

Because this manual serves as a reference, it does not provide general instruction on SQL or relational database concepts. It also does not teach you how to use your operating system or command-line interpreter.

The MySQL Database Software is under constant development, and the Reference Manual is updated frequently as well. The most recent version of the manual is available online in searchable form at Other formats also are available there, including downloadable HTML and PDF versions.

The source code for MySQL itself contains internal documentation written using Doxygen. The generated Doxygen content is available It is also possible to generate this content locally from a MySQL source distribution using the instructions at Section 2.9.10, “Generating MySQL Doxygen Documentation Content”.

If you have questions about using MySQL, join the MySQL Community Slack, or ask in our forums; see MySQL Community Support at the MySQL Forums. If you have suggestions concerning additions or corrections to the manual itself, please send them to the

Typographical and Syntax Conventions

This manual uses certain typographical conventions:

  • Text in this style is used for SQL statements; database, table, and column names; program listings and source code; and environment variables. Example: To reload the grant tables, use the FLUSH PRIVILEGES statement.

  • Text in this style indicates input that you type in examples.

  • Text in this style indicates the names of executable programs and scripts, examples being mysql (the MySQL command-line client program) and mysqld (the MySQL server executable).

  • Text in this style is used for variable input for which you should substitute a value of your own choosing.

  • Text in this style is used for emphasis.

  • Text in this style is used in table headings and to convey especially strong emphasis.

  • Text in this style is used to indicate a program option that affects how the program is executed, or that supplies information that is needed for the program to function in a certain way. Example: The --host option (short form -h) tells the mysql client program the hostname or IP address of the MySQL server that it should connect to.

  • File names and directory names are written like this: The global my.cnf file is located in the /etc directory.

  • Character sequences are written like this: To specify a wildcard, use the % character.

When commands are shown that are meant to be executed from within a particular program, the prompt shown preceding the command indicates which command to use. For example, shell> indicates a command that you execute from your login shell, root-shell> is similar but should be executed as root, and mysql> indicates a statement that you execute from the mysql client program:

shell> type a shell command here
root-shell> type a shell command as root here
mysql> type a mysql statement here

In some areas different systems may be distinguished from each other to show that commands should be executed in two different environments. For example, while working with replication the commands might be prefixed with source and replica:

source> type a mysql command on the replication source here
replica> type a mysql command on the replica here

The shell is your command interpreter. On Unix, this is typically a program such as sh, csh, or bash. On Windows, the equivalent program is or cmd.exe, typically run in a console window.

When you enter a command or statement shown in an example, do not type the prompt shown in the example.

Database, table, and column names must often be substituted into statements. To indicate that such substitution is necessary, this manual uses db_name, tbl_name, and col_name. For example, you might see a statement like this:

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this:

mysql> SELECT author_name FROM biblio_db.author_list;

SQL keywords are not case-sensitive and may be written in any lettercase. This manual uses uppercase.

In syntax descriptions, square brackets ([ and ]) indicate optional words or clauses. For example, in the following statement, IF EXISTS is optional:


When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (|). When one member from a set of choices may be chosen, the alternatives are listed within square brackets ([ and ]):


When one member from a set of choices must be chosen, the alternatives are listed within braces ({ and }):

{DESCRIBE | DESC} tbl_name [col_name | wild]

An ellipsis (...) indicates the omission of a section of a statement, typically to provide a shorter version of more complex syntax. For example, SELECT ... INTO OUTFILE is shorthand for the form of SELECT statement that has an INTO OUTFILE clause following other parts of the statement.

An ellipsis can also indicate that the preceding syntax element of a statement may be repeated. In the following example, multiple reset_option values may be given, with each of those after the first preceded by commas:

RESET reset_option [,reset_option] ...

Commands for setting shell variables are shown using Bourne shell syntax. For example, the sequence to set the CC environment variable and run the configure command looks like this in Bourne shell syntax:

shell> CC=gcc ./configure

If you are using csh or tcsh, you must issue commands somewhat differently:

shell> setenv CC gcc
shell> ./configure

Manual Authorship

The Reference Manual source files are written in DocBook XML format. The HTML version and other formats are produced automatically, primarily using the DocBook XSL stylesheets. For information about DocBook, see

This manual was originally written by David Axmark and Michael Monty Widenius. It is maintained by the MySQL Documentation Team, consisting of Chris Cole, Paul DuBois, Margaret Fisher, Edward Gilmore, Stefan Hinz, David Moss, Philip Olson, Daniel Price, Daniel So, and Jon Stephens.

1.2 Overview of the MySQL Database Management System

1.2.1 What is MySQL?

MySQL, the most popular Open Source SQL database management system, is developed, distributed, and supported by Oracle Corporation.

The MySQL website ( provides the latest information about MySQL software.

  • MySQL is a database management system.

    A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as standalone utilities, or as parts of other applications.

  • MySQL databases are relational.

    A relational database stores data in separate tables rather than putting all the data in one big storeroom. The database structures are organized into physical files optimized for speed. The logical model, with objects such as databases, tables, views, rows, and columns, offers a flexible programming environment. You set up rules governing the relationships between different data fields, such as one-to-one, one-to-many, unique, required or optional, and pointers between different tables. The database enforces these rules, so that with a well-designed database, your application never sees inconsistent, duplicate, orphan, out-of-date, or missing data.

    The SQL part of MySQL stands for Structured Query Language. SQL is the most common standardized language used to access databases. Depending on your programming environment, you might enter SQL directly (for example, to generate reports), embed SQL statements into code written in another language, or use a language-specific API that hides the SQL syntax.

    SQL is defined by the ANSI/ISO SQL Standard. The SQL standard has been evolving since 1986 and several versions exist. In this manual, SQL-92 refers to the standard released in 1992, SQL:1999 refers to the standard released in 1999, and SQL:2003 refers to the current version of the standard. We use the phrase the SQL standard to mean the current version of the SQL Standard at any time.

  • MySQL software is Open Source.

    Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License),, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application, you can buy a commercially licensed version from us. See the MySQL Licensing Overview for more information (

  • The MySQL Database Server is very fast, reliable, scalable, and easy to use.

    If that is what you are looking for, you should give it a try. MySQL Server can run comfortably on a desktop or laptop, alongside your other applications, web servers, and so on, requiring little or no attention. If you dedicate an entire machine to MySQL, you can adjust the settings to take advantage of all the memory, CPU power, and I/O capacity available. MySQL can also scale up to clusters of machines, networked together.

    MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Although under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.

  • MySQL Server works in client/server or embedded systems.

    The MySQL Database Software is a client/server system that consists of a multithreaded SQL server that supports different back ends, several different client programs and libraries, administrative tools, and a wide range of application programming interfaces (APIs).

    We also provide MySQL Server as an embedded multithreaded library that you can link into your application to get a smaller, faster, easier-to-manage standalone product.

  • A large amount of contributed MySQL software is available.

    MySQL Server has a practical set of features developed in close cooperation with our users. It is very likely that your favorite application or language supports the MySQL Database Server.

The official way to pronounce MySQL is My Ess Que Ell (not my sequel), but we do not mind if you pronounce it as my sequel or in some other localized way.

1.2.2 The Main Features of MySQL

This section describes some of the important characteristics of the MySQL Database Software. In most respects, the roadmap applies to all versions of MySQL. For information about features as they are introduced into MySQL on a series-specific basis, see the In a Nutshell section of the appropriate Manual:

Internals and Portability

  • Written in C and C++.

  • Tested with a broad range of different compilers.

  • Works on many different platforms. See

  • For portability, configured using CMake.

  • Tested with Purify (a commercial memory leakage detector) as well as with Valgrind, a GPL tool (

  • Uses multi-layered server design with independent modules.

  • Designed to be fully multithreaded using kernel threads, to easily use multiple CPUs if they are available.

  • Provides transactional and nontransactional storage engines.

  • Uses very fast B-tree disk tables (MyISAM) with index compression.

  • Designed to make it relatively easy to add other storage engines. This is useful if you want to provide an SQL interface for an in-house database.

  • Uses a very fast thread-based memory allocation system.

  • Executes very fast joins using an optimized nested-loop join.

  • Implements in-memory hash tables, which are used as temporary tables.

  • Implements SQL functions using a highly optimized class library that should be as fast as possible. Usually there is no memory allocation at all after query initialization.

  • Provides the server as a separate program for use in a client/server networked environment, and as a library that can be embedded (linked) into standalone applications. Such applications can be used in isolation or in environments where no network is available.

Data Types

Statements and Functions

  • Full operator and function support in the SELECT list and WHERE clause of queries. For example:

    mysql> SELECT CONCAT(first_name, ' ', last_name)
        -> FROM citizen
        -> WHERE income/dependents > 10000 AND age > 30;
  • Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), AVG(), STD(), SUM(), MAX(), MIN(), and GROUP_CONCAT()).

  • Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with both standard SQL and ODBC syntax.

  • Support for aliases on tables and columns as required by standard SQL.

  • Support for DELETE, INSERT, REPLACE, and UPDATE to return the number of rows that were changed (affected), or to return the number of rows matched instead by setting a flag when connecting to the server.

  • Support for MySQL-specific SHOW statements that retrieve information about databases, storage engines, tables, and indexes. Support for the INFORMATION_SCHEMA database, implemented according to standard SQL.

  • An EXPLAIN statement to show how the optimizer resolves a query.

  • Independence of function names from table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are permitted between the function name and the ( that follows it. See Section 9.3, “Keywords and Reserved Words”.

  • You can refer to tables from different databases in the same statement.


  • A privilege and password system that is very flexible and secure, and that enables host-based verification.

  • Password security by encryption of all password traffic when you connect to a server.

Scalability and Limits

  • Support for large databases. We use MySQL Server with databases that contain 50 million records. We also know of users who use MySQL Server with 200,000 tables and about 5,000,000,000 rows.

  • Support for up to 64 indexes per table. Each index may consist of 1 to 16 columns or parts of columns. The maximum index width for InnoDB tables is either 767 bytes or 3072 bytes. See Section 15.22, “InnoDB Limits”. The maximum index width for MyISAM tables is 1000 bytes. See Section 16.2, “The MyISAM Storage Engine”. An index may use a prefix of a column for CHAR, VARCHAR, BLOB, or TEXT column types.


  • Clients can connect to MySQL Server using several protocols:

    • Clients can connect using TCP/IP sockets on any platform.

    • On Windows systems, clients can connect using named pipes if the server is started with the named_pipe system variable enabled. Windows servers also support shared-memory connections if started with the shared_memory system variable enabled. Clients can connect through shared memory by using the --protocol=memory option.

    • On Unix systems, clients can connect using Unix domain socket files.

  • MySQL client programs can be written in many languages. A client library written in C is available for clients written in C or C++, or for any language that provides C bindings.

  • APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl are available, enabling MySQL clients to be written in many languages. See Chapter 29, Connectors and APIs.

  • The Connector/ODBC (MyODBC) interface provides MySQL support for client programs that use ODBC (Open Database Connectivity) connections. For example, you can use MS Access to connect to your MySQL server. Clients can be run on Windows or Unix. Connector/ODBC source is available. All ODBC 2.5 functions are supported, as are many others. See MySQL Connector/ODBC Developer Guide.

  • The Connector/J interface provides MySQL support for Java client programs that use JDBC connections. Clients can be run on Windows or Unix. Connector/J source is available. See MySQL Connector/J 5.1 Developer Guide.

  • MySQL Connector/NET enables developers to easily create .NET applications that require secure, high-performance data connectivity with MySQL. It implements the required ADO.NET interfaces and integrates into ADO.NET aware tools. Developers can build applications using their choice of .NET languages. MySQL Connector/NET is a fully managed ADO.NET driver written in 100% pure C#. See MySQL Connector/NET Developer Guide.


  • The server can provide error messages to clients in many languages. See Section 10.12, “Setting the Error Message Language”.

  • Full support for several different character sets, including latin1 (cp1252), german, big5, ujis, several Unicode character sets, and more. For example, the Scandinavian characters å, ä and ö are permitted in table and column names.

  • All data is saved in the chosen character set.

  • Sorting and comparisons are done according to the default character set and collation. is possible to change this when the MySQL server is started (see Section 10.3.2, “Server Character Set and Collation”). To see an example of very advanced sorting, look at the Czech sorting code. MySQL Server supports many different character sets that can be specified at compile time and runtime.

  • The server time zone can be changed dynamically, and individual clients can specify their own time zone. See Section 5.1.15, “MySQL Server Time Zone Support”.

Clients and Tools

  • MySQL includes several client and utility programs. These include both command-line programs such as mysqldump and mysqladmin, and graphical programs such as MySQL Workbench.

  • MySQL Server has built-in support for SQL statements to check, optimize, and repair tables. These statements are available from the command line through the mysqlcheck client. MySQL also includes myisamchk, a very fast command-line utility for performing these operations on MyISAM tables. See Chapter 4, MySQL Programs.

  • MySQL programs can be invoked with the --help or -? option to obtain online assistance.

1.2.3 History of MySQL

We started out with the intention of using the mSQL database system to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing, we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was designed to enable third-party code that was written for use with mSQL to be ported easily for use with MySQL.

MySQL is named after co-founder Monty Widenius's daughter, My.

The name of the MySQL Dolphin (our logo) is Sakila, which was chosen from a huge list of names suggested by users in our Name the Dolphin contest. The winning name was submitted by Ambrose Twebaze, an Open Source software developer from Eswatini (formerly Swaziland), Africa. According to Ambrose, the feminine name Sakila has its roots in SiSwati, the local language of Eswatini. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

1.3 What Is New in MySQL 8.0

This section summarizes what has been added to, deprecated in, and removed from MySQL 8.0. A companion section lists MySQL server options and variables that have been added, deprecated, or removed in MySQL 8.0; see Section 1.4, “Server and Status Variables and Options Added, Deprecated, or Removed in MySQL 8.0”.

Features Added in MySQL 8.0

The following features have been added to MySQL 8.0:

  • Data dictionary.  MySQL now incorporates a transactional data dictionary that stores information about database objects. In previous MySQL releases, dictionary data was stored in metadata files and nontransactional tables. For more information, see Chapter 14, MySQL Data Dictionary.

  • Atomic data definition statements (Atomic DDL).  An atomic DDL statement combines the data dictionary updates, storage engine operations, and binary log writes associated with a DDL operation into a single, atomic transaction. For more information, see Section 13.1.1, “Atomic Data Definition Statement Support”.

  • Upgrade procedure.  Previously, after installation of a new version of MySQL, the MySQL server automatically upgrades the data dictionary tables at the next startup, after which the DBA is expected to invoke mysql_upgrade manually to upgrade the system tables in the mysql schema, as well as objects in other schemas such as the sys schema and user schemas.

    As of MySQL 8.0.16, the server performs the tasks previously handled by mysql_upgrade. After installation of a new MySQL version, the server now automatically performs all necessary upgrade tasks at the next startup and is not dependent on the DBA invoking mysql_upgrade. In addition, the server updates the contents of the help tables (something mysql_upgrade did not do). A new --upgrade server option provides control over how the server performs automatic data dictionary and server upgrade operations. For more information, see Section 2.11.3, “What the MySQL Upgrade Process Upgrades”.

  • Security and account management.  These enhancements were added to improve security and enable greater DBA flexibility in account management:

    • The grant tables in the mysql system database are now InnoDB (transactional) tables. Previously, these were MyISAM (nontransactional) tables. The change of grant table storage engine underlies an accompanying change to the behavior of account-management statements. Previously, an account-management statement (such as CREATE USER or DROP USER) that named multiple users could succeed for some users and fail for others. Now, each statement is transactional and either succeeds for all named users or rolls back and has no effect if any error occurs. The statement is written to the binary log if it succeeds, but not if it fails; in that case, rollback occurs and no changes are made. For more information, see Section 13.1.1, “Atomic Data Definition Statement Support”.

    • A new caching_sha2_password authentication plugin is available. Like the sha256_password plugin, caching_sha2_password implements SHA-256 password hashing, but uses caching to address latency issues at connect time. It also supports more transport protocols and does not require linking against OpenSSL for RSA key pair-based password-exchange capabilities. See Section, “Caching SHA-2 Pluggable Authentication”.

      The caching_sha2_password and sha256_password authentication plugins provide more secure password encryption than the mysql_native_password plugin, and caching_sha2_password provides better performance than sha256_password. Due to these superior security and performance characteristics of caching_sha2_password, it is now the preferred authentication plugin, and is also the default authentication plugin rather than mysql_native_password. For information about the implications of this change of default plugin for server operation and compatibility of the server with clients and connectors, see caching_sha2_password as the Preferred Authentication Plugin.

    • MySQL now supports roles, which are named collections of privileges. Roles can be created and dropped. Roles can have privileges granted to and revoked from them. Roles can be granted to and revoked from user accounts. The active applicable roles for an account can be selected from among those granted to the account, and can be changed during sessions for that account. For more information, see Section 6.2.10, “Using Roles”.

    • MySQL now incorporates the concept of user account categories, with system and regular users distinguished according to whether they have the SYSTEM_USER privilege. See Section 6.2.11, “Account Categories”.

    • Previously, it was not possible to grant privileges that apply globally except for certain schemas. This is now possible if the partial_revokes system variable is enabled. See Section 6.2.12, “Privilege Restriction Using Partial Revokes”.

    • The GRANT statement has an AS user [WITH ROLE] clause that specifies additional information about the privilege context to use for statement execution. This syntax is visible at the SQL level, although its primary purpose is to enable uniform replication across all nodes of grantor privilege restrictions imposed by partial revokes, by causing those restrictions to appear in the binary log. See Section, “GRANT Statement”.

    • MySQL now maintains information about password history, enabling restrictions on reuse of previous passwords. DBAs can require that new passwords not be selected from previous passwords for some number of password changes or period of time. It is possible to establish password-reuse policy globally as well as on a per-account basis.

      It is now possible to require that attempts to change account passwords be verified by specifying the current password to be replaced. This enables DBAs to prevent users from changing password without proving that they know the current password. It is possible to establish password-verification policy globally as well as on a per-account basis.

      Accounts are now permitted to have dual passwords, which enables phased password changes to be performed seamlessly in complex multiple-server systems, without downtime.

      MySQL now enables administrators to configure user accounts such that too many consecutive login failures due to incorrect passwords cause temporary account locking. The required number of failures and the lock time are configurable per account.

      These new capabilities provide DBAs more complete control over password management. For more information, see Section 6.2.15, “Password Management”.

    • MySQL now supports FIPS mode, if compiled using OpenSSL, and an OpenSSL library and FIPS Object Module are available at runtime. FIPS mode imposes conditions on cryptographic operations such as restrictions on acceptable encryption algorithms or requirements for longer key lengths. See Section 6.8, “FIPS Support”.

    • The TLS context the server uses for new connections now is reconfigurable at runtime. This capability may be useful, for example, to avoid restarting a MySQL server that has been running so long that its SSL certificate has expired. See Server-Side Runtime Configuration and Monitoring for Encrypted Connections.

    • OpenSSL 1.1.1 supports the TLS v1.3 protocol for encrypted connections, and MySQL 8.0.16 and higher supports TLS v1.3 as well, if both the server and client are compiled using OpenSSL 1.1.1 or higher. See Section 6.3.2, “Encrypted Connection TLS Protocols and Ciphers”.

    • MySQL now sets the access control granted to clients on the named pipe to the minimum necessary for successful communication on Windows. Newer MySQL client software can open named pipe connections without any additional configuration. If older client software cannot be upgraded immediately, the new named_pipe_full_access_group system variable can be used to give a Windows group the necessary permissions to open a named pipe connection. Membership in the full-access group should be restricted and temporary.

  • Resource management.  MySQL now supports creation and management of resource groups, and permits assigning threads running within the server to particular groups so that threads execute according to the resources available to the group. Group attributes enable control over its resources, to enable or restrict resource consumption by threads in the group. DBAs can modify these attributes as appropriate for different workloads. Currently, CPU time is a manageable resource, represented by the concept of virtual CPU as a term that includes CPU cores, hyperthreads, hardware threads, and so forth. The server determines at startup how many virtual CPUs are available, and database administrators with appropriate privileges can associate these CPUs with resource groups and assign threads to groups. For more information, see Section 5.1.16, “Resource Groups”.

  • Table encryption management.  Table encryption can now be managed globally by defining and enforcing encryption defaults. The default_table_encryption variable defines an encryption default for newly created schemas and general tablespace. The encryption default for a schema can also be defined using the DEFAULT ENCRYPTION clause when creating a schema. By default, a table inherits the encryption of the schema or general tablespace it is created in. Encryption defaults are enforced by enabling the table_encryption_privilege_check variable. The privilege check occurs when creating or altering a schema or general tablespace with an encryption setting that differs from the default_table_encryption setting, or when creating or altering a table with an encryption setting that differs from the default schema encryption. The TABLE_ENCRYPTION_ADMIN privilege permits overriding default encryption settings when table_encryption_privilege_check is enabled. For more information, see Defining an Encryption Default for Schemas and General Tablespaces.

  • InnoDB enhancements.  These InnoDB enhancements were added:

    • The current maximum auto-increment counter value is written to the redo log each time the value changes, and saved to an engine-private system table on each checkpoint. These changes make the current maximum auto-increment counter value persistent across server restarts. Additionally:

      • A server restart no longer cancels the effect of the AUTO_INCREMENT = N table option. If you initialize the auto-increment counter to a specific value, or if you alter the auto-increment counter value to a larger value, the new value is persisted across server restarts.

      • A server restart immediately following a ROLLBACK operation no longer results in the reuse of auto-increment values that were allocated to the rolled-back transaction.

      • If you modify an AUTO_INCREMENT column value to a value larger than the current maximum auto-increment value (in an UPDATE operation, for example), the new value is persisted, and subsequent INSERT operations allocate auto-increment values starting from the new, larger value.

      For more information, see Section, “AUTO_INCREMENT Handling in InnoDB”, and InnoDB AUTO_INCREMENT Counter Initialization.

    • When encountering index tree corruption, InnoDB writes a corruption flag to the redo log, which makes the corruption flag crash safe. InnoDB also writes in-memory corruption flag data to an engine-private system table on each checkpoint. During recovery, InnoDB reads corruption flags from both locations and merges results before marking in-memory table and index objects as corrupt.

    • The InnoDB memcached plugin supports multiple get operations (fetching multiple key-value pairs in a single memcached query) and range queries. See Section 15.20.4, “InnoDB memcached Multiple get and Range Query Support”.

    • A new dynamic variable, innodb_deadlock_detect, may be used to disable deadlock detection. On high concurrency systems, deadlock detection can cause a slowdown when numerous threads wait for the same lock. At times, it may be more efficient to disable deadlock detection and rely on the innodb_lock_wait_timeout setting for transaction rollback when a deadlock occurs.

    • The new INFORMATION_SCHEMA.INNODB_CACHED_INDEXES table reports the number of index pages cached in the InnoDB buffer pool for each index.

    • InnoDB temporary tables are now created in the shared temporary tablespace, ibtmp1.

    • The InnoDB tablespace encryption feature supports encryption of redo log and undo log data. See Redo Log Encryption, and Undo Log Encryption.

    • InnoDB supports NOWAIT and SKIP LOCKED options with SELECT ... FOR SHARE and SELECT ... FOR UPDATE locking read statements. NOWAIT causes the statement to return immediately if a requested row is locked by another transaction. SKIP LOCKED removes locked rows from the result set. See Locking Read Concurrency with NOWAIT and SKIP LOCKED.

      SELECT ... FOR SHARE replaces SELECT ... LOCK IN SHARE MODE, but LOCK IN SHARE MODE remains available for backward compatibility. The statements are equivalent. However, FOR UPDATE and FOR SHARE support NOWAIT, SKIP LOCKED, and OF tbl_name options. See Section 13.2.10, “SELECT Statement”.

      OF tbl_name applies locking queries to named tables.

    • ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REORGANIZE PARTITION, and REBUILD PARTITION ALTER TABLE options are supported by native partitioning in-place APIs and may be used with ALGORITHM={COPY|INPLACE} and LOCK clauses.

      DROP PARTITION with ALGORITHM=INPLACE deletes data stored in the partition and drops the partition. However, DROP PARTITION with ALGORITHM=COPY or old_alter_table=ON rebuilds the partitioned table and attempts to move data from the dropped partition to another partition with a compatible PARTITION ... VALUES definition. Data that cannot be moved to another partition is deleted.

    • The InnoDB storage engine now uses the MySQL data dictionary rather than its own storage engine-specific data dictionary. For information about the data dictionary, see Chapter 14, MySQL Data Dictionary.

    • mysql system tables and data dictionary tables are now created in a single InnoDB tablespace file named mysql.ibd in the MySQL data directory. Previously, these tables were created in individual InnoDB tablespace files in the mysql database directory.

    • The following undo tablespace changes are introduced in MySQL 8.0:

      • By default, undo logs now reside in two undo tablespaces that are created when the MySQL instance is initialized. Undo logs are no longer created in the system tablespace.

      • As of MySQL 8.0.14, additional undo tablespaces can be created in a chosen location at runtime using CREATE UNDO TABLESPACE syntax.

        CREATE UNDO TABLESPACE tablespace_name ADD DATAFILE 'file_name.ibu';

        Undo tablespaces created using CREATE UNDO TABLESPACE syntax can be dropped at runtime using DROP UNDO TABLESPACE syntax.

        DROP UNDO TABLESPACE tablespace_name;

        ALTER UNDO TABLESPACE syntax can be used to mark an undo tablespace as active or inactive.


        A STATE column that shows the state of a tablespace was added to the INFORMATION_SCHEMA.INNODB_TABLESPACES table. An undo tablespace must be in an empty state before it can be dropped.

      • The innodb_undo_log_truncate variable is enabled by default.

      • The innodb_rollback_segments variable defines the number of rollback segments per undo tablespace. Previously, innodb_rollback_segments specified the total number of rollback segments for the MySQL instance. This change increases the number of rollback segments available for concurrent transactions. More rollback segments increases the likelihood that concurrent transactions use separate rollback segments for undo logs, resulting in less resource contention.

    • Default values for variables that affect buffer pool preflushing and flushing behavior were modified:

      • The innodb_max_dirty_pages_pct_lwm default value is now 10. The previous default value of 0 disables buffer pool preflushing. A value of 10 enables preflushing when the percentage of dirty pages in the buffer pool exceeds 10%. Enabling preflushing improves performance consistency.

      • The innodb_max_dirty_pages_pct default value was increased from 75 to 90. InnoDB attempts to flush data from the buffer pool so that the percentage of dirty pages does not exceed this value. The increased default value permits a greater percentage of dirty pages in the buffer pool.

    • The default innodb_autoinc_lock_mode setting is now 2 (interleaved). Interleaved lock mode permits the execution of multi-row inserts in parallel, which improves concurrency and scalability. The new innodb_autoinc_lock_mode default setting reflects the change from statement-based replication to row based replication as the default replication type in MySQL 5.7. Statement-based replication requires the consecutive auto-increment lock mode (the previous default) to ensure that auto-increment values are assigned in a predictable and repeatable order for a given sequence of SQL statements, whereas row-based replication is not sensitive to the execution order of SQL statements. For more information, see InnoDB AUTO_INCREMENT Lock Modes.

      For systems that use statement-based replication, the new innodb_autoinc_lock_mode default setting may break applications that depend on sequential auto-increment values. To restore the previous default, set innodb_autoinc_lock_mode to 1.

    • Renaming a general tablespace is supported by ALTER TABLESPACE ... RENAME TO syntax.

    • The new innodb_dedicated_server variable, which is disabled by default, can be used to have InnoDB automatically configure the following options according to the amount of memory detected on the server:

      This option is intended for MySQL server instances that run on a dedicated server. For more information, see Section 15.8.12, “Enabling Automatic Configuration for a Dedicated MySQL Server”.

    • The new INFORMATION_SCHEMA.INNODB_TABLESPACES_BRIEF view provides space, name, path, flag, and space type data for InnoDB tablespaces.

    • The zlib library version bundled with MySQL was raised from version 1.2.3 to version 1.2.11. MySQL implements compression with the help of the zlib library.

      If you use InnoDB compressed tables, see Section 2.11.4, “Changes in MySQL 8.0” for related upgrade implications.

    • Serialized dictionary information (SDI) is present in all InnoDB tablespace files except for global temporary tablespace and undo tablespace files. SDI is serialized metadata for table and tablespace objects. The presence of SDI data provides metadata redundancy. For example, dictionary object metadata may be extracted from tablespace files if the data dictionary becomes unavailable. SDI extraction is performed using the ibd2sdi tool. SDI data is stored in JSON format.

      The inclusion of SDI data in tablespace files increases tablespace file size. An SDI record requires a single index page, which is 16KB in size by default. However, SDI data is compressed when it is stored to reduce the storage footprint.

    • The InnoDB storage engine now supports atomic DDL, which ensures that DDL operations are either fully committed or rolled back, even if the server halts during the operation. For more information, see Section 13.1.1, “Atomic Data Definition Statement Support”.

    • Tablespace files can be moved or restored to a new location while the server is offline using the innodb_directories option. For more information, see Section, “Moving Tablespace Files While the Server is Offline”.

    • The following redo logging optimizations were implemented:

      • User threads can now write concurrently to the log buffer without synchronizing writes.

      • User threads can now add dirty pages to the flush list in a relaxed order.

      • A dedicated log thread is now responsible for writing the log buffer to the system buffers, flushing system buffers to disk, notifying user threads about written and flushed redo, maintaining the lag required for the relaxed flush list order, and write checkpoints.

      • System variables were added for configuring the use of spin delay by user threads waiting for flushed redo:

      • The innodb_log_buffer_size variable is now dynamic, which permits resizing of the log buffer while the server is running.

      For more information, see Section 8.5.4, “Optimizing InnoDB Redo Logging”.

    • As of MySQL 8.0.12, undo logging is supported for small updates to large object (LOB) data, which improves performance of LOB updates that are 100 bytes in size or less. Previously, LOB updates were a minimum of one LOB page in size, which is less than optimal for updates that might only modify a few bytes. This enhancement builds upon support added in MySQL 8.0.4 for partial update of LOB data.

    • As of MySQL 8.0.12, ALGORITHM=INSTANT is supported for the following ALTER TABLE operations:

      • Adding a column. This feature is also referred to as Instant ADD COLUMN. Limitations apply. See Section 15.12.1, “Online DDL Operations”.

      • Adding or dropping a virtual column.

      • Adding or dropping a column default value.

      • Modifying the definition of an ENUM or SET column.

      • Changing the index type.

      • Renaming a table.

      Operations that support ALGORITHM=INSTANT only modify metadata in the data dictionary. No metadata locks are taken on the table, and table data is unaffected, making the operations instantaneous. If not specified explicitly, ALGORITHM=INSTANT is used by default by operations that support it. If ALGORITHM=INSTANT is specified but not supported, the operation fails immediately with an error.

      For more information about operations that support ALGORITHM=INSTANT, see Section 15.12.1, “Online DDL Operations”.

    • As of MySQL 8.0.13, the TempTable storage engine supports storage of binary large object (BLOB) type columns. This enhancement improves performance for queries that use temporary tables containing BLOB data. Previously, temporary tables that contained BLOB data were stored in the on-disk storage engine defined by internal_tmp_disk_storage_engine. For more information, see Section 8.4.4, “Internal Temporary Table Use in MySQL”.

    • As of MySQL 8.0.13, the InnoDB data-at-rest encryption feature supports general tablespaces. Previously, only file-per-table tablespaces could be encrypted. To support encryption of general tablespaces, CREATE TABLESPACE and ALTER TABLESPACE syntax was extended to include an ENCRYPTION clause.

      The INFORMATION_SCHEMA.INNODB_TABLESPACES table now includes an ENCRYPTION column that indicates whether or not a tablespace is encrypted.

      The stage/innodb/alter tablespace (encryption) Performance Schema stage instrument was added to permit monitoring of general tablespace encryption operations.

    • Disabling the innodb_buffer_pool_in_core_file variable reduces the size of core files by excluding InnoDB buffer pool pages. To use this variable, the core_file variable must be enabled and the operating system must support the MADV_DONTDUMP non-POSIX extension to madvise(), which is supported in Linux 3.4 and later. For more information, see Section, “Excluding Buffer Pool Pages from Core Files”.

    • As of MySQL 8.0.13, user-created temporary tables and internal temporary tables created by the optimizer are stored in session temporary tablespaces that are allocated to a session from a pool of temporary tablespaces. When a session disconnects, its temporary tablespaces are truncated and released back to the pool. In previous releases, temporary tables were created in the global temporary tablespace (ibtmp1), which did not return disk space to the operating system after temporary tables were dropped.

      The innodb_temp_tablespaces_dir variable defines the location where session temporary tablespaces are created. The default location is the #innodb_temp directory in the data directory.

      The INNODB_SESSION_TEMP_TABLESPACES table provides metadata about session temporary tablespaces.

      The global temporary tablespace (ibtmp1) now stores rollback segments for changes made to user-created temporary tables.

    • As of MySQL 8.0.14, InnoDB supports parallel clustered index reads, which can improve CHECK TABLE performance. This feature does not apply to secondary index scans. The innodb_parallel_read_threads session variable must be set to a value greater than 1 for parallel clustered index reads to occur. The default value is 4. The actual number of threads used to perform a parallel clustered index read is determined by the innodb_parallel_read_threads setting or the number of index subtrees to scan, whichever is smaller.

    • As of 8.0.14, when the innodb_dedicated_server variable is enabled, the size and number of log files are configured according to the automatically configured buffer pool size. Previously, log file size was configured according to the amount of memory detected on the server, and the number of log files was not configured automatically.

    • As of 8.0.14, the ADD DATAFILE clause of the CREATE TABLESPACE statement is optional, which permits users without the FILE privilege to create tablespaces. A CREATE TABLESPACE statement executed without an ADD DATAFILE clause implicitly creates a tablespace data file with a unique file name.

    • By default, when the amount of memory occupied by the TempTable storage engine exceeds the memory limit defined by the temptable_max_ram variable, the TempTable storage engine begins allocating memory-mapped temporary files from disk. As of MySQL 8.0.16, this behavior is controlled by the temptable_use_mmap variable. Disabling temptable_use_mmap causes the TempTable storage engine to use InnoDB on-disk internal temporary tables instead of memory-mapped files as its overflow mechanism. For more information, see Internal Temporary Table Storage Engine.

    • As of MySQL 8.0.16, the InnoDB data-at-rest encryption feature supports encryption of the mysql system tablespace. The mysql system tablespace contains the mysql system database and the MySQL data dictionary tables. For more information, see Section 15.13, “InnoDB Data-at-Rest Encryption”.

    • The innodb_spin_wait_pause_multiplier variable, introduced in MySQL 8.0.16, provides greater control over the duration of spin-lock polling delays that occur when a thread waits to acquire a mutex or rw-lock. Delays can be tuned more finely to account for differences in PAUSE instruction duration on different processor architectures. For more information, see Section 15.8.8, “Configuring Spin Lock Polling”.

    • InnoDB parallel read thread performance for large data sets was improved in MySQL 8.0.17 through better utilization of read threads, through a reduction in read thread I/O for prefetch activity that occurs during parallel scans, and through support for parallel scanning of partitions.

      The parallel read thread feature is controlled by the innodb_parallel_read_threads variable. The maximum setting is now 256, which is the total number of threads for all client connections. If the thread limit is reached, connections fall back to using a single thread.

    • The innodb_idle_flush_pct variable, introduced in MySQL 8.0.18, permits placing a limit on page flushing during idle periods, which can help extend the life of solid state storage devices. See Limiting Buffer Flushing During Idle Periods.

    • Efficient sampling of InnoDB data for the purpose of generating histogram statistics is supported as of MySQL 8.0.19. See Histogram Statistics Analysis.

    • As of MySQL 8.0.20, the doublewrite buffer storage area resides in doublewrite files. In previous releases, the storage area resided in the system tablespace. Moving the storage area out of the system tablespace reduces write latency, increases throughput, and provides flexibility with respect to placement of doublewrite buffer pages. The following system variables were introduced for advanced doublewrite buffer configuration:

      For more information, see Section 15.6.4, “Doublewrite Buffer”.

    • The Contention-Aware Transaction Scheduling (CATS) algorithm, which prioritizes transactions that are waiting for locks, was improved in MySQL 8.0.20. Transaction scheduling weight computation is now performed a separate thread entirely, which improves computation performance and accuracy.

      The First In First Out (FIFO) algorithm, which had also been used for transaction scheduling, was removed. The FIFO algorithm was rendered redundant by CATS algorithm enhancements. Transaction scheduling previously performed by the FIFO algorithm is now performed by the CATS algorithm.

      A TRX_SCHEDULE_WEIGHT column was added to the INFORMATION_SCHEMA.INNODB_TRX table, which permits querying transaction scheduling weights assigned by the CATS algorithm.

      The following INNODB_METRICS counters were added for monitoring code-level transaction scheduling events:

      • lock_rec_release_attempts

        The number of attempts to release record locks.

      • lock_rec_grant_attempts

        The number of attempts to grant record locks.

      • lock_schedule_refreshes

        The number of times the wait-for graph was analyzed to update transaction schedule weights.

      For more information, see Section 15.7.6, “Transaction Scheduling”.

    • As of MySQL 8.0.21, to improve concurrency for operations that require access to lock queues for table and row resources, the lock system mutex (lock_sys->mutex) was replaced in by sharded latches, and lock queues were grouped into table and page lock queue shards, with each shard protected by a dedicated mutex. Previously, the single lock system mutex protected all lock queues, which was a point of contention on high-concurrency systems. The new sharded implementation permits more granular access to lock queues.

      The lock system mutex (lock_sys->mutex) was replaced by the following sharded latches:

      • A global latch (lock_sys->latches.global_latch) consisting of 64 read-write lock objects (rw_lock_t). Access to an individual lock queue requires a shared global latch and a latch on the lock queue shard. Operations that require access to all lock queues take an exclusive global latch, which latches all table and page lock queue shards.

      • Table shard latches (lock_sys->latches.table_shards.mutexes), implemented as an array of 512 mutexes, with each mutex dedicated to one of 512 table lock queue shards.

      • Page shard latches (lock_sys->latches.page_shards.mutexes), implemented as an array of 512 mutexes, with each mutex dedicated to one of 512 page lock queue shards.

      The Performance Schema wait/synch/mutex/innodb/lock_mutex instrument for monitoring the single lock system mutex was replaced by instruments for monitoring the new global, table shard, and page shard latches:

      • wait/synch/sxlock/innodb/lock_sys_global_rw_lock

      • wait/synch/mutex/innodb/lock_sys_table_mutex

      • wait/synch/mutex/innodb/lock_sys_page_mutex

    • As of MySQL 8.0.21, table and table partition data files created outside of the data directory using the DATA DIRECTORY clause are restricted to directories known to InnoDB. This change permits database administrators to control where tablespace data files are created and ensures that the data files can be found during recovery.

      General and file-per-table tablespaces data files (.ibd files) can no longer be created in the undo tablespace directory (innodb_undo_directory) unless that directly is known to InnoDB.

      Known directories are those defined by the datadir, innodb_data_home_dir, and innodb_directories variables.

      Truncating an InnoDB table that resides in a file-per-table tablespace drops the existing tablespace and creates a new one. As of MySQL 8.0.21, InnoDB creates the new tablespace in the default location and writes a warning to the error log if the current tablespace directory is unknown. To have TRUNCATE TABLE create the tablespace in its current location, add the directory to the innodb_directories setting before running TRUNCATE TABLE.

    • As of MySQL 8.0.21, redo logging can be enabled and disabled using ALTER INSTANCE {ENABLE|DISABLE} INNODB REDO_LOG syntax. This functionality is intended for loading data into a new MySQL instance. Disabling redo logging helps speed up data loading by avoiding redo log writes.

      The new INNODB_REDO_LOG_ENABLE privilege permits enabling and disabling redo logging.

      The new Innodb_redo_log_enabled status variable permits monitoring redo logging status.

      See Disabling Redo Logging.

    • At startup, InnoDB validates the paths of known tablespace files against tablespace file paths stored in the data dictionary in case tablespace files have been moved to a different location. The new innodb_validate_tablespace_paths variable, introduced in MySQL 8.0.21, permits disabling tablespace path validation. This feature is intended for environments where tablespaces files are not moved. Disabling tablespace path validation improves startup time on systems with a large number of tablespace files.

      For more information, see Section, “Disabling Tablespace Path Validation”.

    • As of MySQL 8.0.21, on storage engines that support atomic DDL, the CREATE TABLE ... SELECT statement is logged as one transaction in the binary log when row-based replication is in use. Previously, it was logged as two transactions, one to create the table, and the other to insert data. With this change, CREATE TABLE ... SELECT statements are now safe for row-based replication and permitted for use with GTID-based replication. For more information, see Section 13.1.1, “Atomic Data Definition Statement Support”.

    • Truncating an undo tablespace on a busy system could affect performance due to associated flushing operations that remove old undo tablespace pages from the buffer pool and flush the initial pages of the new undo tablespace to disk. To address this issue, the flushing operations are removed as of MySQL 8.0.21.

      Old undo tablespace pages are released passively as they become least recently used, or are removed at the next full checkpoint. The initial pages of the new undo tablespace are now redo logged instead of flushed to disk during the truncate operation, which also improves durability of the undo tablespace truncate operation.

      To prevent potential issues caused by an excessive number of undo tablespace truncate operations, truncate operations on the same undo tablespace between checkpoints are now limited to 64. If the limit is exceeded, an undo tablespace can still be made inactive, but it is not truncated until after the next checkpoint.

      INNODB_METRICS counters associated with defunct undo truncate flushing operations were removed. Removed counters include: undo_truncate_sweep_count, undo_truncate_sweep_usec, undo_truncate_flush_count, and undo_truncate_flush_usec.

      See Section, “Undo Tablespaces”.

    • As of MySQL 8.0.22, the new innodb_extend_and_initialize variable permits configuring how InnoDB allocates space to file-per-table and general tablespaces on Linux. By default, when an operation requires additional space in a tablespace, InnoDB allocates pages to the tablespace and physically writes NULLs to those pages. This behavior affects performance if new pages are allocated frequently. You can disable innodb_extend_and_initialize on Linux systems to avoid physically writing NULLs to newly allocated tablespace pages. When innodb_extend_and_initialize is disabled, space is allocated using posix_fallocate() calls, which reserve space without physically writing NULLs.

      A posix_fallocate() operation is not atomic, which makes it possible for a failure to occur between allocating space to a tablespace file and updating the file metadata. Such a failure can leave newly allocated pages in an uninitialized state, resulting in a failure when InnoDB attempts to access those pages. To prevent this scenario, InnoDB writes a redo log record before allocating a new tablespace page. If a page allocation operation is interrupted, the operation is replayed from the redo log record during recovery.

    • As of MySQL 8.0.23, InnoDB supports encryption of doublewrite file pages belonging to encrypted tablespaces. The pages are encrypted using the encryption key of the associated tablespace. For more information, see Section 15.13, “InnoDB Data-at-Rest Encryption”.

    • The temptable_max_mmap variable, introduced in MySQL 8.0.23, defines the maximum amount of memory the TempTable storage engine is permitted to allocate from memory-mapped (MMAP) files before it starts storing internal temporary table data on disk. A setting of 0 disables allocation from MMAP files. For more information, see Section 8.4.4, “Internal Temporary Table Use in MySQL”.

    • The AUTOEXTEND_SIZE option, introduced in MySQL 8.0.23, defines the amount by which InnoDB extends the size of a tablespace when it becomes full, making it possible to extend tablespace size in larger increments. The AUTOEXTEND_SIZE option is supported with the CREATE TABLE, ALTER TABLE, CREATE TABLESPACE, and ALTER TABLESPACE statements. For more information, see Section, “Tablespace AUTOEXTEND_SIZE Configuration”.

      An AUTOEXTEND_SIZE size column was added to the INFORMATION_SCHEMA.INNODB_TABLESPACES table.

  • Character set support.  The default character set has changed from latin1 to utf8mb4. The utf8mb4 character set has several new collations, including utf8mb4_ja_0900_as_cs, the first Japanese language-specific collation available for Unicode in MySQL. For more information, see Section 10.10.1, “Unicode Character Sets”.

  • JSON enhancements.  The following enhancements or additions were made to MySQL's JSON functionality:

    • Added the ->> (inline path) operator, which is equivalent to calling JSON_UNQUOTE() on the result of JSON_EXTRACT().

      This is a refinement of the column path operator -> introduced in MySQL 5.7; col->>"$.path" is equivalent to JSON_UNQUOTE(col->"$.path"). The inline path operator can be used wherever you can use JSON_UNQUOTE(JSON_EXTRACT()), such SELECT column lists, WHERE and HAVING clauses, and ORDER BY and GROUP BY clauses. For more information, see the description of the operator, as well as JSON Path Syntax.

    • Added two JSON aggregation functions JSON_ARRAYAGG() and JSON_OBJECTAGG(). JSON_ARRAYAGG() takes a column or expression as its argument, and aggregates the result as a single JSON array. The expression can evaluate to any MySQL data type; this does not have to be a JSON value. JSON_OBJECTAGG() takes two columns or expressions which it interprets as a key and a value; it returns the result as a single JSON object. For more information and examples, see Section 12.20, “Aggregate Functions”.

    • Added the JSON utility function JSON_PRETTY(), which outputs an existing JSON value in an easy-to-read format; each JSON object member or array value is printed on a separate line, and a child object or array is intended 2 spaces with respect to its parent.

      This function also works with a string that can be parsed as a JSON value.

      For more detailed information and examples, see Section 12.18.8, “JSON Utility Functions”.

    • When sorting JSON values in a query using ORDER BY, each value is now represented by a variable-length part of the sort key, rather than a part of a fixed 1K in size. In many cases this can reduce excessive usage. For example, a scalar INT or even BIGINT value actually requires very few bytes, so that the remainder of this space (up to 90% or more) was taken up by padding. This change has the following benefits for performance:

      • Sort buffer space is now used more effectively, so that filesorts need not flush to disk as early or often as with fixed-length sort keys. This means that more data can be sorted in memory, avoiding unnecessary disk access.

      • Shorter keys can be compared more quickly than longer ones, providing a noticeable improvement in performance. This is true for sorts performed entirely in memory as well as for sorts that require writing to and reading from disk.

    • Added support in MySQL 8.0.2 for partial, in-place updates of JSON column values, which is more efficient than completely removing an existing JSON value and writing a new one in its place, as was done previously when updating any JSON column. For this optimization to be applied, the update must be applied using JSON_SET(), JSON_REPLACE(), or JSON_REMOVE(). New elements cannot be added to the JSON document being updated; values within the document cannot take more space than they did before the update. See Partial Updates of JSON Values, for a detailed discussion of the requirements.

      Partial updates of JSON documents can be written to the binary log, taking up less space than logging complete JSON documents. Partial updates are always logged as such when statement-based replication is in use. For this to work with row-based replication, you must first set binlog_row_value_options=PARTIAL_JSON; see this variable's description for more information.

    • Added the JSON utility functions JSON_STORAGE_SIZE() and JSON_STORAGE_FREE(). JSON_STORAGE_SIZE() returns the storage space in bytes used for the binary representation of a JSON document prior to any partial update (see previous item). JSON_STORAGE_FREE() shows the amount of space remaining in a table column of type JSON after it has been partially updated using JSON_SET() or JSON_REPLACE(); this is greater than zero if the binary representation of the new value is less than that of the previous value.

      Each of these functions also accepts a valid string representation of a JSON document. For such a value, JSON_STORAGE_SIZE() returns the space used by its binary representation following its conversion to a JSON document. For a variable containing the string representation of a JSON document, JSON_STORAGE_FREE() returns zero. Either function produces an error if its (non-null) argument cannot be parsed as a valid JSON document, and NULL if the argument is NULL.

      For more information and examples, see Section 12.18.8, “JSON Utility Functions”.

      JSON_STORAGE_SIZE() and JSON_STORAGE_FREE() were implemented in MySQL 8.0.2.

    • Added support in MySQL 8.0.2 for ranges such as $[1 to 5] in XPath expressions. Also added support in this version for the last keyword and relative addressing, such that $[last] always selects the last (highest-numbered) element in the array and $[last-1] the next to last element. last and expressions using it can also be included in range definitions. For example, $[last-2 to last-1] returns the last two elements but one from an array. See Searching and Modifying JSON Values, for additional information and examples.

    • Added a JSON merge function intended to conform to RFC 7396. JSON_MERGE_PATCH(), when used on 2 JSON objects, merges them into a single JSON object that has as members a union of the following sets:

      • Each member of the first object for which there is no member with the same key in the second object.

      • Each member of the second object for which there is no member having the same key in the first object, and whose value is not the JSON null literal.

      • Each member having a key that exists in both objects, and whose value in the second object is not the JSON null literal.

      As part of this work, the JSON_MERGE() function has been renamed JSON_MERGE_PRESERVE(). JSON_MERGE() continues to be recognized as an alias for JSON_MERGE_PRESERVE() in MySQL 8.0, but is now deprecated and is subject to removal in a future version of MySQL.

      For more information and examples, see Section 12.18.4, “Functions That Modify JSON Values”.

    • Implemented last duplicate key wins normalization of duplicate keys, consistent with RFC 7159 and most JavaScript parsers. An example of this behavior is shown here, where only the rightmost member having the key x is preserved:

      mysql> SELECT JSON_OBJECT('x', '32', 'y', '[true, false]',
           >                     'x', '"abc"', 'x', '100') AS Result;
      | Result                             |
      | {"x": "100", "y": "[true, false]"} |
      1 row in set (0.00 sec)

      Values inserted into MySQL JSON columns are also normalized in this way, as shown in this example:

      mysql> CREATE TABLE t1 (c1 JSON);
      mysql> INSERT INTO t1 VALUES ('{"x": 17, "x": "red", "x": [3, 5, 7]}');
      mysql> SELECT c1 FROM t1;
      | c1               |
      | {"x": [3, 5, 7]} |

      This is an incompatible change from previous versions of MySQL, where a first duplicate key wins algorithm was used in such cases.

      See Normalization, Merging, and Autowrapping of JSON Values, for more information and examples.

    • Added the JSON_TABLE() function in MySQL 8.0.4. This function accepts JSON data and returns it as a relational table having the specified columns.

      This function has the syntax JSON_TABLE(expr, path COLUMNS column_list) [AS] alias), where expr is an expression that returns JSON data, path is a JSON path applied to the source, and column_list is a list of column definitions. An example is shown here:

      mysql> SELECT *
          -> FROM
          ->   JSON_TABLE(
          ->     '[{"a":3,"b":"0"},{"a":"3","b":"1"},{"a":2,"b":1},{"a":0},{"b":[1,2]}]',
          ->     "$[*]" COLUMNS(
          ->       rowid FOR ORDINALITY,
          ->       xa INT EXISTS PATH "$.a",
          ->       xb INT EXISTS PATH "$.b",
          ->       sa VARCHAR(100) PATH "$.a",
          ->       sb VARCHAR(100) PATH "$.b",
          ->       ja JSON PATH "$.a",
          ->       jb JSON PATH "$.b"
          ->     )
          ->   ) AS  jt1;
      | rowid | xa   | xb   | sa   | sb   | ja   | jb     |
      |     1 |    1 |    1 | 3    | 0    | 3    | "0"    |
      |     2 |    1 |    1 | 3    | 1    | "3"  | "1"    |
      |     3 |    1 |    1 | 2    | 1    | 2    | 1      |
      |     4 |    1 |    0 | 0    | NULL | 0    | NULL   |
      |     5 |    0 |    1 | NULL | NULL | NULL | [1, 2] |

      The JSON source expression can be any expression that yields a valid JSON document, including a JSON literal, a table column, or a function call that returns JSON such as JSON_EXTRACT(t1, data, '$.post.comments'). For more information, see Section 12.18.6, “JSON Table Functions”.

  • Data type support.  MySQL now supports use of expressions as default values in data type specifications. This includes the use of expressions as default values for the BLOB, TEXT, GEOMETRY, and JSON data types, which previously could not be assigned default values at all. For details, see Section 11.6, “Data Type Default Values”.

  • Optimizer.  These optimizer enhancements were added:

    • MySQL now supports invisible indexes. An invisible index is not used by the optimizer at all, but is otherwise maintained normally. Indexes are visible by default. Invisible indexes make it possible to test the effect of removing an index on query performance, without making a destructive change that must be undone should the index turn out to be required. See Section 8.3.12, “Invisible Indexes”.

    • MySQL now supports descending indexes: DESC in an index definition is no longer ignored but causes storage of key values in descending order. Previously, indexes could be scanned in reverse order but at a performance penalty. A descending index can be scanned in forward order, which is more efficient. Descending indexes also make it possible for the optimizer to use multiple-column indexes when the most efficient scan order mixes ascending order for some columns and descending order for others. See Section 8.3.13, “Descending Indexes”.

    • MySQL now supports creation of functional index key parts that index expression values rather than column values. Functional key parts enable indexing of values that cannot be indexed otherwise, such as JSON values. For details, see Section 13.1.15, “CREATE INDEX Statement”.

    • In MySQL 8.0.14 and later, trivial WHERE conditions arising from constant literal expressions are removed during preparation, rather than later on during optimization. Removal of the condition earlier in the process makes it possible to simplify joins for queries with outer joins having trivial conditions, such as this one:

      SELECT * FROM t1 LEFT JOIN t2 ON condition_1 WHERE condition_2 OR 0 = 1

      The optimizer now sees during preparation that 0 = 1 is always false, making OR 0 = 1 redundant, and removes it, leaving this:

      SELECT * FROM t1 LEFT JOIN t2 ON condition_1 where condition_2

      Now the optimizer can rewrite the query as an inner join, like this:

      SELECT * FROM t1 LEFT JOIN t2 WHERE condition_1 AND condition_2

      For more information, see Section, “Outer Join Optimization”.

    • In MySQL 8.0.16 and later, MySQL can use constant folding at optimization time to handle comparisons between a column and a constant value where the constant is out of range or on a range boundary with respect to the type of the column, rather than doing so for each row at execution time. For example, given a table t with a TINYINT UNSIGNED column c, the optimizer can rewrite a condition such as WHERE c < 256 to WHERE 1 (and optimize the condition away altogether), or WHERE c >= 255 to WHERE c = 255.

      See Section, “Constant-Folding Optimization”, for more information.

    • Beginning with MySQL 8.0.16, the semijoin optimizations used with IN subqueries can now be applied to EXISTS subqueries as well. In addition, the optimizer now decorrelates trivially-correlated equality predicates in the WHERE condition attached to the subquery, so that they can be treated similarly to expressions in IN subqueries; this applies to both EXISTS and IN subqueries.

      For more information, see Section, “Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations”.

    • As of MySQL 8.0.17, the server rewrites any incomplete SQL predicates (that is, predicates having the form WHERE value, in which value is a column name or constant expression and no comparison operator is used) internally as WHERE value <> 0 during the contextualization phase, so that the query resolver, query optimizer, and query executor need work only with complete predicates.

      One visible effect of this change is that, for Boolean values, EXPLAIN output now shows true and false, rather than 1 and 0.

      Another effect of this change is that evaluation of a JSON value in an SQL boolean context performs an implicit comparison against JSON integer 0. Consider the table created and populated as shown here:

      mysql> CREATE TABLE test (id INT, col JSON);
      mysql> INSERT INTO test VALUES (1, '{"val":true}'), (2, '{"val":false}');

      Previously, the server attempted to convert an extracted true or false value to an SQL boolean when comparing it in an SQL boolean context, as shown by the following query using IS TRUE:

      mysql> SELECT id, col, col->"$.val" FROM test WHERE col->"$.val" IS TRUE;
      | id   | col           | col->"$.val" |
      |    1 | {"val": true} | true         |

      In MySQL 8.0.17 and later, the implicit copmparison of the extracted value with JSON integer 0 leads to a different result:

      mysql> SELECT id, col, col->"$.val" FROM test WHERE col->"$.val" IS TRUE;
      | id   | col            | col->"$.val" |
      |    1 | {"val": true}  | true         |
      |    2 | {"val": false} | false        |

      Beginning with MySQL 8.0.21, you can use JSON_VALUE() on the extracted value to perform type conversion prior to performing the test, as shown here:

      mysql> SELECT id, col, col->"$.val" FROM test
          ->     WHERE JSON_VALUE(col, "$.val" RETURNING UNSIGNED) IS TRUE;
      | id   | col           | col->"$.val" |
      |    1 | {"val": true} | true         |

      Also beginning with MySQL 8.0.21, the server provides the warning Evaluating a JSON value in SQL boolean context does an implicit comparison against JSON integer 0; if this is not what you want, consider converting JSON to an SQL numeric type with JSON_VALUE RETURNING when comparing extracted values in an SQL boolean context in this manner.

    • In MySQL 8.0.17 and later a WHERE condition having NOT IN (subquery) or NOT EXISTS (subquery) is transformed internally into an antijoin. (An antijoin returns all rows from the table for which there is no row in the table to which it is joined matching the join condition.) This removes the subquery which can result in faster query execution since the subquery's tables are now handled on the top level.

      This is similar to, and reuses, the existing IS NULL (Not exists) optimization for outer joins; see EXPLAIN Extra Information.

    • Beginning with MySQL 8.0.21, a single-table UPDATE or DELETE statement can now in many cases make use of a semijoin transformation or subquery materialization. This applies to statements of the forms shown here:

      • UPDATE t1 SET t1.a=value WHERE t1.a IN (SELECT t2.a FROM t2)

      • DELETE FROM t1 WHERE t1.a IN (SELECT t2.a FROM t2)

      This can be done for a single-table UPDATE or DELETE meeting the following conditions:

      • The UPDATE or DELETE statement uses a subquery having a [NOT] IN or [NOT] EXISTS predicate.

      • The statement has no ORDER BY clause, and has no LIMIT clause.

        (The multi-table versions of UPDATE and DELETE do not support ORDER BY or LIMIT.)

      • The target table does not support read-before-write removal (relevant only for NDB tables).

      • Semijoin or subquery materialization is allowed, based on any hints contained in the subquery and the value of optimizer_switch.

      When the semijoin optimization is used for an eligible single-table DELETE or UPDATE, this is visible in the optimizer trace: for a multi-table statement there is a join_optimization object in the trace, while there is none for a single-table statement. The conversion is also visible in the output of EXPLAIN FORMAT=TREE or EXPLAIN ANALYZE; a single-table statement shows <not executable by iterator executor>, while a multi-table statement reports a full plan.

      Alo beginning with MySQL 8.0.21, semi-consistent reads are supported by multi-table UPDATE statements using InnoDB tables, for transaction isolation levels weaker than REPEATABLE READ.

    • Improved hash join performance.  MySQL 8.0.23 reimplements the hash table used for hash joins, resulting in several improvements in hash join performance. This work includes a fix for an issue (Bug #31516149, Bug #99933) whereby only roughly 2/3 of the memory allocated for the join buffer (join_buffer_size) could actually be used by a hash join.

      The new hash table is generally faster than the old one, and uses less memory for alignment, keys/values, and in scenarios where there are many equal keys. In addition, the server can now free old memory when the size of the hash table increases.

  • Common table expressions.  MySQL now supports common table expressions, both nonrecursive and recursive. Common table expressions enable use of named temporary result sets, implemented by permitting a WITH clause preceding SELECT statements and certain other statements. For more information, see Section 13.2.15, “WITH (Common Table Expressions)”.

    As of MySQL 8.0.19, the recursive SELECT part of a recursive common table expression (CTE) supports a LIMIT clause. LIMIT with OFFSET is also supported. See Recursive Common Table Expressions, for more information.

  • Window functions.  MySQL now supports window functions that, for each row from a query, perform a calculation using rows related to that row. These include functions such as RANK(), LAG(), and NTILE(). In addition, several existing aggregate functions now can be used as window functions (for example, SUM() and AVG()). For more information, see Section 12.21, “Window Functions”.

  • Lateral derived tables.  A derived table now may be preceded by the LATERAL keyword to specify that it is permitted to refer to (depend on) columns of preceding tables in the same FROM clause. Lateral derived tables make possible certain SQL operations that cannot be done with nonlateral derived tables or that require less-efficient workarounds. See Section, “Lateral Derived Tables”.

  • Aliases in single-table DELETE statements.  In MySQL 8.0.16 and later, single-table DELETE statements support the use of table aliases.

  • Regular expression support.  Previously, MySQL used the Henry Spencer regular expression library to support regular expression operators (REGEXP, RLIKE). Regular expression support has been reimplemented using International Components for Unicode (ICU), which provides full Unicode support and is multibyte safe. The REGEXP_LIKE() function performs regular expression matching in the manner of the REGEXP and RLIKE operators, which now are synonyms for that function. In addition, the REGEXP_INSTR(), REGEXP_REPLACE(), and REGEXP_SUBSTR() functions are available to find match positions and perform substring substitution and extraction, respectively. The regexp_stack_limit and regexp_time_limit system variables provide control over resource consumption by the match engine. For more information, see Section 12.8.2, “Regular Expressions”. For information about ways in which applications that use regular expressions may be affected by the implementation change, see Regular Expression Compatibility Considerations.

  • Internal temporary tables.  The TempTable storage engine replaces the MEMORY storage engine as the default engine for in-memory internal temporary tables. The TempTable storage engine provides efficient storage for VARCHAR and VARBINARY columns. The internal_tmp_mem_storage_engine session variable defines the storage engine for in-memory internal temporary tables. Permitted values are TempTable (the default) and MEMORY. The temptable_max_ram variable defines the maximum amount of memory that the TempTable storage engine can use before data is stored to disk.

  • Logging.  Error logging was rewritten to use the MySQL component architecture. Traditional error logging is implemented using built-in components, and logging using the system log is implemented as a loadable component. In addition, a loadable JSON log writer is available. To control which log components to enable, use the log_error_services system variable. For more information, see Section 5.4.2, “The Error Log”.

  • Backup lock.  A new type of backup lock permits DML during an online backup while preventing operations that could result in an inconsistent snapshot. The new backup lock is supported by LOCK INSTANCE FOR BACKUP and UNLOCK INSTANCE syntax. The BACKUP_ADMIN privilege is required to use these statements.

  • Replication.  The following enhancements have been made to MySQL Replication:

    • MySQL Replication now supports binary logging of partial updates to JSON documents using a compact binary format, saving space in the log over logging complete JSON documents. Such compact logging is done automatically when statement-based logging is in use, and can be enabled by setting the new binlog_row_value_options system variable to PARTIAL_JSON. For more information, see Partial Updates of JSON Values, as well as the description of binlog_row_value_options.

  • Connection management.  MySQL Server now permits a TCP/IP port to be configured specifically for administrative connections. This provides an alternative to the single administrative connection that is permitted on the network interfaces used for ordinary connections even when max_connections connections are already established. See Section, “Connection Interfaces”.

    MySQL now provides more control over the use of compression to minimize the number of bytes sent over connections to the server. Previously, a given connection was either uncompressed or used the zlib compression algorithm. Now, it is also possible to use the zstd algorithm, and to select a compression level for zstd connections. The permitted compression algorithms can be configured on the server side, as well as on the connection-origination side for connections by client programs and by servers participating in source/replica replication or Group Replication. For more information, see Section 4.2.8, “Connection Compression Control”.

  • Configuration.  The maximum permitted length of host names throughout MySQL has been raised to 255 ASCII characters, up from the previous limit of 60 characters. This applies to, for example, host name-related columns in the data dictionary, mysql system schema, Performance Schema, INFORMATION_SCHEMA, and sys schema; the MASTER_HOST value for the CHANGE MASTER TO statement; the Host column in SHOW PROCESSLIST statement output; host names in account names (such as used in account-management statements and in DEFINER attributes); and host name-related command options and system variables.


    • The increase in permitted host name length can affect tables with indexes on host name columns. For example, tables in the mysql system schema that index host names now have an explicit ROW_FORMAT attribute of DYNAMIC to accommodate longer index values.

    • Some file name-valued configuration settings might be constructed based on the server host name. The permitted values are constrained by the underlying operating system, which may not permit file names long enough to include 255-character host names. This affects the general_log_file, log_error, pid_file, relay_log, and slow_query_log_file system variables and corresponding options. If host name-based values are too long for the OS, explicit shorter values must be provided.

    • Although the server now supports 255-character host names, connections to the server established using the --ssl-mode=VERIFY_IDENTITY option are constrained by maximum host name length supported by OpenSSL. Host name matches pertain to two fields of SSL certificates, which have maximum lengths as follows: Common Name: maximum length 64; Subject Alternative Name: maximum length as per RFC#1034.

  • Plugins.  Previously, MySQL plugins could be written in C or C++. MySQL header files used by plugins now contain C++ code, which means that plugins must be written in C++, not C.

  • C API.  The MySQL C API now supports asynchronous functions for nonblocking communication with the MySQL server. Each function is the asynchronous counterpart to an existing synchronous function. The synchronous functions block if reads from or writes to the server connection must wait. The asynchronous functions enable an application to check whether work on the server connection is ready to proceed. If not, the application can perform other work before checking again later. See C API Asynchronous Interface.

  • Additional target types for casts.  The functions CAST() and CONVERT() now support conversions to types DOUBLE, FLOAT, and REAL. Added in MySQL 8.0.17. See Section 12.11, “Cast Functions and Operators”.

  • JSON schema validation.  MySQL 8.0.17 adds two functions JSON_SCHEMA_VALID() and JSON_SCHEMA_VALIDATION_REPORT() for validating JSON documents again JSON schemas. JSON_SCHEMA_VALID() returns TRUE (1) if the document validates against the schema and FALSE (0) if it does not. JSON_SCHEMA_VALIDATION_REPORT() returns a JSON document containing detailed information about the results of the validation. The following statements apply to both of these functions:

    • The schema must conform to Draft 4 of the JSON Schema specification.

    • required attributes are supported.

    • External resources and the $ref keyword are not supported.

    • Regular expression patterns are supported; invalid patterns are silently ignored.

    See Section 12.18.7, “JSON Schema Validation Functions”, for more information and examples.

  • Multi-valued indexes.  Beginning with MySQL 8.0.17, InnoDB supports the creation of a multi-valued index, which is a secondary index defined on a JSON column that stores an array of values and which can have multiple index records for a single data record. Such an index uses a key part definition such as CAST(data->'$.zipcode' AS UNSIGNED ARRAY). A multi-valued index is used automatically by the MySQL optimizer for suitable queries, as can be viewed in the output of EXPLAIN.

    As part of this work, MySQL adds a new function JSON_OVERLAPS() and a new MEMBER OF() operator for working with JSON documents, additionally extending the CAST() function with a new ARRAY keyword, as described in the following list:

    • JSON_OVERLAPS() compares two JSON documents. If they contain any key-value pairs or array elements in common, the function returns TRUE (1); otherwise it returns FALSE (0). If both values are scalars, the function performs a simple test for equality. If one argument is a JSON array and the other is a scalar, the scalar is treated as an array element. Thus, JSON_OVERLAPS() acts as a complement to JSON_CONTAINS().

    • MEMBER OF() tests whether the first operand (a scalar or JSON document) is a member of the JSON array passed as the second operand, returning TRUE (1) if it is, and FALSE (0) if it is not. No type conversion of the operand is performed.

    • CAST(expression AS type ARRAY) permits creation of a functional index by casting the JSON array found in a JSON document at json_path to an SQL array. Type specifiers are limited to those already supported by CAST(), with the exception of BINARY (not supported). This usage of CAST() (and the ARRAY keyword) is supported only by InnoDB, and only for the creation of a multi-valued index.

    For detailed information about multi-valued indexes, including examples, see Multi-Valued Indexes. Section 12.18.3, “Functions That Search JSON Values”, provides information about JSON_OVERLAPS() and MEMBER OF(), along with examples of use.

  • Hintable time_zone.  As of MySQL 8.0.17, the time_zone session variable is hintable using SET_VAR.

  • Redo Log Archiving.  As of MySQL 8.0.17, InnoDB supports redo log archiving. Backup utilities that copy redo log records may sometimes fail to keep pace with redo log generation while a backup operation is in progress, resulting in lost redo log records due to those records being overwritten. The redo log archiving feature addresses this issue by sequentially writing redo log records to an archive file. Backup utilities can copy redo log records from the archive file as necessary, thereby avoiding the potential loss of data. For more information, see Redo Log Archiving.

  • The Clone Plugin.  As of MySQL 8.0.17, MySQL provides a clone plugin that permits cloning InnoDB data locally or from a remote MySQL server instance. A local cloning operation stores cloned data on the same server or node where the MySQL instance runs. A remote cloning operation transfers cloned data over the network from a donor MySQL server instance to the recipient server or node where the cloning operation was initiated.

    The clone plugin supports replication. In addition to cloning data, a cloning operation extracts and transfers replication coordinates from the donor and applies them on the recipient, which enables using the clone plugin for provisioning Group Replication members and replicas. Using the clone plugin for provisioning is considerably faster and more efficient than replicating a large number of transactions. Group Replication members can also be configured to use the clone plugin as an alternative method of recovery, so that members automatically choose the most efficient way to retrieve group data from seed members.

    For more information, see Section 5.6.7, “The Clone Plugin”, and Section, “Cloning for Distributed Recovery”.

  • Hash Join Optimization.  Beginning with MySQL 8.0.18, a hash join is used whenever each pair of tables in a join includes at least one equi-join condition, and no indexes apply to any join condition. A hash join does not require indexes, although it can be used with indexes applying to single-table predicates only. A hash join is more efficient in most cases than the block-nested loop algorithm. Joins such as those shown here can be optimized in this manner:

    SELECT *
        FROM t1
        JOIN t2
            ON t1.c1=t2.c1;
    SELECT *
        FROM t1
        JOIN t2
            ON (t1.c1 = t2.c1 AND t1.c2 < t2.c2)
        JOIN t3
            ON (t2.c1 = t3.c1)

    Hash joins can also be used for Cartesian products—that is, when no join condition is specified.

    You can see when the hash join optimization is being used for a particular query using EXPLAIN FORMAT=TREE or EXPLAIN ANALYZE. (In MySQL 8.0.20 and later, you can also use EXPLAIN, omitting FORMAT=TREE.)

    The amount of memory available to a hash join is limited by the value of join_buffer_size. A hash join that requires more than this much memory is executed on disk; the number of disk files that can be used by an on-disk hash join is limited by open_files_limit.

    As of MySQL 8.0.19, the hash_join optimizer switch which was introduced in MySQL 8.0.18 no longer supported (hash_join=on still appears as part of the value of optimizer_switch, but setting it no longer has any effect). The HASH_JOIN and NO_HASH_JOIN optimizer hints are also no longer supported. The switch and the hint are both now deprecated; expect them to be removed in a future MySQL release. In MySQL 8.0.18 and later, hash joins can be disabled using the NO_BNL optimizer switch.

    In MySQL 8.0.20 and later, block nested loop is no longer used in the MySQL server, and a hash join is employed any time a block nested loop would have been used previously, even when the query contains no equi-join conditions. This applies to inner non-equijoins, semijoins, antijoins, left outer joins, and right outer joins. TThe block_nested_loop flag for the optimizer_switch system variable as well as the BNL and NO_BNL optimizer hints are still supported, but henceforth control use of hash joins only. In addition, both inner and outer joins (including semijoins and antijoins) can now employ batched key access (BKA), which allocates join buffer memory incrementally so that individual queries need not use up large amounts of resources that they do not actually require for resolution. BKA for inner joins only is supported starting with MySQL 8.0.18.

    MySQL 8.0.20 also replaces the executor used in previous versions of MySQL with the iterator executor. This work includes replacement of the old index subquery engines that governed queries of the form WHERE value IN (SELECT column FROM table WHERE ...) for those IN queries which have not been optimized as semijoins, as well as queries materialized in the same form, which formerly depended on the old executor.

    For more information and examples, see Section, “Hash Join Optimization”. See also Batched Key Access Joins.

  • EXPLAIN ANALYZE Statement.  A new form of the EXPLAIN statement, EXPLAIN ANALYZE, is implemented in MySQL 8.0.18, providing expanded information about the execution of SELECT statements in TREE format for each iterator used in processing the query, and making it possible to compare estimated cost with the actual cost of the query. This information includes startup cost, total cost, number of rows returned by this iterator, and the number of loops executed.

    In MySQL 8.0.21 and later, this statement also supports a FORMAT=TREE specifier. TREE is the only supported format.

    See Obtaining Information with EXPLAIN ANALYZE, for more information.

  • Query cast injection.  In version 8.0.18 and later, MySQL injects cast operations into the query item tree inside expressions and conditions in which the data type of the argument and the expected data type do not match. This has no effect on query results or speed of execution, but makes the query as executed equivalent to one which is compliant with the SQL standard while maintaining backwards compatibility with previous releases of MySQL.

    Such implicit casts are now performed between temporal types (DATE, DATETIME, TIMESTAMP, TIME) and numeric types (SMALLINT, TINYINT, MEDIUMINT, INT/INTEGER, BIGINT; DECIMAL/NUMERIC; FLOAT, DOUBLE, REAL; BIT) whenever they are compared using any of the standard numeric comparison operators (=, >=, >, <, <=, <>/!=, or <=>). In this case, any value that is not already a DOUBLE is cast as one. Cast injection is also now performed for comparisons between DATE or TIME values and DATETIME values, where the arguments are cast whenever necessary as DATETIME.

    Beginning with MySQL 8.0.21, such casts are also performed when comparing string types with other types. String types that are cast include CHAR, VARCHAR, BINARY, VARBINARY, BLOB, TEXT, ENUM, and SET. When comparing a value of a string type with a numeric type or YEAR, the string cast is to DOUBLE; if the type of the other argument is not FLOAT, DOUBLE, or REAL, it is also cast to DOUBLE. When comparing a string type to a DATETIME or TIMESTAMP value, the string is cast is to DATETIME; when comparing a string type with DATE, the string is cast to DATE.

    It is possible to see when casts are injected into a given query by viewing the output of EXPLAIN ANALYZE, EXPLAIN FORMAT=JSON, or, as shown here, EXPLAIN FORMAT=TREE:

    mysql> CREATE TABLE d (dt DATETIME, d DATE, t TIME);
    Query OK, 0 rows affected (0.62 sec)
    mysql> CREATE TABLE n (i INT, d DECIMAL, f FLOAT, dc DECIMAL);
    Query OK, 0 rows affected (0.51 sec)
    mysql> CREATE TABLE s (c CHAR(25), vc VARCHAR(25),
        ->     bn BINARY(50), vb VARBINARY(50), b BLOB, t TEXT,
        ->     e ENUM('a', 'b', 'c'), se SET('x' ,'y', 'z'));
    Query OK, 0 rows affected (0.50 sec)
    mysql> EXPLAIN FORMAT=TREE SELECT * from d JOIN n ON d.dt = n.i\G
    *************************** 1. row ***************************
    EXPLAIN: -> Inner hash join (cast(d.dt as double) = cast(n.i as double))
    (cost=0.70 rows=1)
        -> Table scan on n  (cost=0.35 rows=1)
        -> Hash
            -> Table scan on d  (cost=0.35 rows=1)
    mysql> EXPLAIN FORMAT=TREE SELECT * from s JOIN d ON d.dt = s.c\G
    *************************** 1. row ***************************
    EXPLAIN: -> Inner hash join (d.dt = cast(s.c as datetime(6)))  (cost=0.72 rows=1)
        -> Table scan on d  (cost=0.37 rows=1)
        -> Hash
            -> Table scan on s  (cost=0.35 rows=1)
    1 row in set (0.01 sec)
    mysql> EXPLAIN FORMAT=TREE SELECT * from n JOIN s ON n.d = s.c\G
    *************************** 1. row ***************************
    EXPLAIN: -> Inner hash join (cast(n.d as double) = cast(s.c as double))  (cost=0.70 rows=1)
        -> Table scan on s  (cost=0.35 rows=1)
        -> Hash
            -> Table scan on n  (cost=0.35 rows=1)
    1 row in set (0.00 sec)

    Such casts can also be seen by executing EXPLAIN [FORMAT=TRADITIONAL], in which case it is also necessary to issue SHOW WARNINGS after executing the EXPLAIN statement.

  • Time zone support for TIMESTAMP and DATETIME.  As of MySQL 8.0.19, the server accepts a time zone offset with inserted datetime (TIMESTAMP and DATETIME) values. This offset uses the same format as that employed when setting the time_zone system variable, except that a leading zero is required when the hours portion of the offset is less than 10, and '-00:00' is not allowed. Examples of datetime literals that include time zone offsets are '2019-12-11 10:40:30-05:00', '2003-04-14 03:30:00+10:00', and '2020-01-01 15:35:45+05:30'.

    Time zone offsets are not displayed when selecting datetime values.

    Datetime literals incorporating time zone offsets can be used as prepared statement parameter values.

    As part of this work, the value used to set the time_zone system variable is now also restricted to the range -14:00 to +14:00, inclusive. (It remains possible to assign name values to time_zone such as 'EST', 'Posix/Australia/Brisbane', and 'Europe/Stockholm' to this variable, provided that the MySQL time zone tables are loaded; see Populating the Time Zone Tables).

    For more information and examples, see Section 5.1.15, “MySQL Server Time Zone Support”, as well as Section 11.2.2, “The DATE, DATETIME, and TIMESTAMP Types”.

  • Precise information for JSON schema CHECK constraint failures.  When using JSON_SCHEMA_VALID() to specify a CHECK constraint, MySQL 8.0.19 and later provides precise information about the reasons for failures of such constraints.

    For examples and more information, see JSON_SCHEMA_VALID() and CHECK constraints. See also Section, “CHECK Constraints”.

  • Row and column aliases with ON DUPLICATE KEY UPDATE.  Beginning with MySQL 8.0.19, it is possible to reference the row to be inserted, and, optionally, its columns, using aliases. Consider the following INSERT statement on a table t having columns a and b:

    INSERT INTO t SET a=9,b=5

    Using the alias new for the new row, and, in some cases, the aliases m and n for this row's columns, the INSERT statement can be rewritten in many different ways, some examples of which are shown here:

    INSERT INTO t SET a=9,b=5 AS new
        ON DUPLICATE KEY UPDATE a=new.a+new.b;
    INSERT INTO t VALUES(9,5) AS new
        ON DUPLICATE KEY UPDATE a=new.a+new.b;
    INSERT INTO t SET a=9,b=5 AS new(m,n)
    INSERT INTO t VALUES(9,5) AS new(m,n)

    For more information and examples, see Section, “INSERT ... ON DUPLICATE KEY UPDATE Statement”.

  • SQL standard explicit table clause and table value constructor.  Added table value constructors and explicit table clauses according to the SQL standard. These are implemented in MySQL 8.0.19, respectively, as the TABLE statement and the VALUES statement.

    The TABLE statement has the format TABLE table_name, and is equivalent to SELECT * FROM table_name. It supports ORDER BY and LIMIT clauses ( the latter with optional OFFSET), but does not allow for the selection of individual table columns. TABLE can be used anywhere that you would employ the equivalent SELECT statement; this includes joins, unions, INSERT ... SELECT, REPLACE, CREATE TABLE ... SELECT statements, and subqueries. For example:

    • TABLE t1 UNION TABLE t2 is equivalent to SELECT * FROM t1 UNION SELECT * FROM t2

    • CREATE TABLE t2 TABLE t1 is equivalent to CREATE TABLE t2 SELECT * FROM t1

    • SELECT a FROM t1 WHERE b > ANY (TABLE t2) is equivalent to SELECT a FROM t1 WHERE b > ANY (SELECT * FROM t2).

    VALUES can be used to supply a table value to an INSERT, REPLACE, or SELECT statement, and consists of the VALUES keyword followed by a series of row constructors (ROW()) separated by commas. For example, the statement INSERT INTO t1 VALUES ROW(1,2,3), ROW(4,5,6), ROW(7,8,9) provides an SQL-compliant equivalent to the MySQL-specific INSERT INTO t1 VALUES (1,2,3), (4,5,6), (7,8,9). You can also select from a VALUES table value constructor just as you would a table, bearing in mind that you must supply a table alias when doing so, and use this SELECT just as you would any other; this includes joins, unions, and subqueries.

    For more information about TABLE and VALUES, and for examples of their use, see the following sections of this documentation:

  • Optimizer hints for FORCE INDEX, IGNORE INDEX.  MySQL 8.0 introduces index-level optimizer hints which serve as analogs to the traditional index hints as described in Section 8.9.4, “Index Hints”. The new hints are listed here, along with their FORCE INDEX or IGNORE INDEX equivalents:

    For example, the following two queries are equivalent:

    SELECT /*+ JOIN_INDEX(t1 i_a) */ a FROM t1 WHERE a=1 AND b=2;

    The optimizer hints listed previously follow the same basic rules for syntax and usage as existing index-level optimizer hints.

    These optimizer hints are intended to replace FORCE INDEX and IGNORE INDEX, which we plan to deprecate in a future MySQL release, and subsequently to remove from MySQL. They do not implement a single exact equivalent for USE INDEX; instead, you can employ one or more of NO_INDEX, NO_JOIN_INDEX, NO_GROUP_INDEX, or NO_ORDER_INDEX to achieve the same effect.

    For further information and examples of use, see Index-Level Optimizer Hints.

  • JSON_VALUE() function.  MySQL 8.0.21 implements a new function JSON_VALUE() intended to simplify indexing of JSON columns. In its most basic form, it takes as arguments a JSON document and a JSON path pointing to a single value in that document, as well as (optionally) allowing you to specify a return type with the RETURNING keyword. JSON_VALUE(json_doc, path RETURNING type) is equivalent to this:

        JSON_UNQUOTE( JSON_EXTRACT(json_doc, path) )
        AS type

    You can also specify ON EMPTY, ON ERROR, or both clauses, similar to those employed with JSON_TABLE().

    You can use JSON_VALUE() to create an index on an expression on a JSON column like this:

        j JSON,
        INDEX i1 ( (JSON_VALUE(j, '$.id' RETURNING UNSIGNED)) )
    INSERT INTO t1 VALUES ROW('{"id": "123", "name": "shoes", "price": "49.95"}');

    A query using this expression, such as that shown here, can make use of the index:

    SELECT name, price FROM t1

    In many cases, this is simpler than creating a generated column from the JSON column and then creating an index on the generated column.

    For more information and examples, see the description of JSON_VALUE().

  • User comments and user attributes.  MySQL 8.0.21 introduces the ability to set user comments and user attributes when creating or updating user accounts. A user comment consists of arbitrary text passed as the argument to a COMMENT clause used with a CREATE USER or ALTER USER statement. A user attribute consists of data in the form of a JSON object passed as the argument to an ATTRIBUTE clause used with either of these two statements. The attribute can contain any valid key-value pairs in JSON object notation. Only one of COMMENT or ATTRIBUTE can be used in a single CREATE USER or ALTER USER statement.

    User comments and user attributes are stored together internally as a JSON object, the comment text as the value of an element having comment as its key. This information can be retrieved from the ATTRIBUTE column of the INFORMATION_SCHEMA.USER_ATTRIBUTES table; since it is in JSON format, you can use MySQL's JSON function and operators to parse its contents (see Section 12.18, “JSON Functions”). Successive changes to the user attribute are merged with its current value as when using the JSON_MERGE_PATCH() function.


    mysql> CREATE USER 'mary'@'localhost' COMMENT 'This is Mary Smith\'s account';
    Query OK, 0 rows affected (0.33 sec)
    mysql> ALTER USER 'mary'@'localhost'
        -≫     ATTRIBUTE '{"fname":"Mary", "lname":"Smith"}';
    Query OK, 0 rows affected (0.14 sec)
    mysql> ALTER USER 'mary'@'localhost'
        -≫     ATTRIBUTE '{"email":""}';
    Query OK, 0 rows affected (0.12 sec)
    mysql> SELECT
        ->    USER,
        ->    HOST,
        ->    ATTRIBUTE->>"$.fname" AS 'First Name',
        ->    ATTRIBUTE->>"$.lname" AS 'Last Name',
        ->    ATTRIBUTE->>"$.email" AS 'Email',
        ->    ATTRIBUTE->>"$.comment" AS 'Comment'
        -> WHERE USER='mary' AND HOST='localhost'\G
    *************************** 1. row ***************************
          USER: mary
          HOST: localhost
    First Name: Mary
     Last Name: Smith
       Comment: This is Mary Smith's account
    1 row in set (0.00 sec)

    For more information and examples, see Section, “CREATE USER Statement”, Section, “ALTER USER Statement”, and Section 26.46, “The INFORMATION_SCHEMA USER_ATTRIBUTES Table”.

  • New optimizer_switch flags.  MySQL 8.0.21 adds two new flags for the optimizer_switch system variable, as described in the following list:

    • prefer_ordering_index flag

      By default, MySQL attempts to use an ordered index for any ORDER BY or GROUP BY query that has a LIMIT clause, whenever the optimizer determines that this would result in faster execution. Because it is possible in some cases that choosing a different optimization for such queries actually performs better, it is now possible to disable this optimization by setting the prefer_ordering_index flag to off.

      The default value for this flag is on.

    • subquery_to_derived flag

      When this flag is set to on, the optimizer transforms eligible scalar subqueries into joins on derived tables. For example, the query SELECT * FROM t1 WHERE t1.a > (SELECT COUNT(a) FROM t2) is rewritten as SELECT t1.a FROM t1 JOIN ( SELECT COUNT(t2.a) AS c FROM t2 ) AS d WHERE t1.a > d.c.

      This optimization can be applied to a subquery which is part of a SELECT, WHERE, JOIN, or HAVING clause; contains one or more aggregate functions but no GROUP BY clause; is not correlated; and does not use any nondeterministic functions.

      The optimization can also be applied to a table subquery which is the argument to IN, NOT IN, EXISTS, or NOT EXISTS, and which does not contain a GROUP BY. For example, the query SELECT * FROM t1 WHERE t1.b < 0 OR t1.a IN (SELECT t2.a + 1 FROM t2) is rewritten as SELECT a, b FROM t1 LEFT JOIN (SELECT DISTINCT 1 AS e1, t2.a AS e2 FROM t2) d ON t1.a + 1 = d.e2 WHERE t1.b < 0 OR d.e1 IS NOT NULL.

      This optimization is normally disabled, as it does not yield a noticeable performance benefit in most cases, and so the flag is set to off by default.

    For more information, see Section 8.9.2, “Switchable Optimizations”. See also Section, “LIMIT Query Optimization”, Section, “Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations”, and Section, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.

  • XML enhancements.  As of MySQL 8.0.21, the LOAD XML statement now supports CDATA sections in the XML to be imported.

  • Casting to the YEAR type now supported.  Beginning with MySQL 8.0.22, the server allows casting to YEAR. Both the CAST() and CONVERT() functions support single-digit, two-digit, and four-digit YEAR values. For one-digit and two-digit values, the allowed range is 0-99. Four-digit values must be in the range 1901-2155. YEAR can also be used as the return type for the JSON_VALUE() function; this function supports four-digit years only.

    String, time-and-date, and floating-point values can all be cast to YEAR. Casting of GEOMETRY values to YEAR is not supported.

    For more information, including conversion rules, see the description of the CONVERT() function.

  • Retrieval of TIMESTAMP values as UTC.  MySQL 8.0.22 and later supports conversion of a TIMESTAMP column value from the system time zone to a UTC DATETIME on retrieval, using CAST(value AT TIME ZONE specifier AS DATETIME), where the specifier is one of [INTERVAL] '+00:00' or 'UTC'. The precision of the DATETIME value returned by the cast can be specified up to 6 decimal places, if desired. The ARRAY keyword is not supported with this construct.

    TIMESTAMP values inserted into a table using a timezone offset are also supported. Use of AT TIME ZONE is not supported for CONVERT() or any other MySQL function or construct.

    For further information and examples, see the description of the CAST() function.

  • Dump file output synchronization.  MySQL 8.0.22 and later supports periodic synchronization when writing to files by SELECT INTO DUMPFILE and SELECT INTO OUTFILE statements. This can be enabled by setting the select_into_disk_sync system variable to ON; the size of the write buffer is determined by the value set for select_into_buffer_size; the default is 131072 (217) bytes.

    In addition, an optional delay following synchronization to disk can be set using select_into_disk_sync_delay; the default is no delay (0 milliseconds).

    For more information, see the descriptions of the variables referenced previously in this item.

  • Single preparation of statements.  As of MySQL 8.0.22, a prepared statement is prepared a single time, rather than once each time it is executed. This is done when executing PREPARE. This is also true for any statement inside a stored procedure; the statement is prepared once, when the stored procedure is first executed.

    One result of this change is that the fashion in which dynamic parameters used in prepared statements are resolved is also changed in the ways listed here:

    • A prepared statement parameter is assigned a data type when the statement is prepared; the type persists for each subsequent execution of the statement (unless the statement is reprepared; see following).

      Using a different data type for a given parameter or user variable within a prepared statement for executions of the statement subsequent to the first execution may cause the statement to be reprepared; for this reason, it is advisable to use the same data type for a given parameter when re-executing a prepared statement.

    • The following constructs employing window functions are no longer accepted, in order to align with the SQL standard:

      This facilitates greater compliance with the SQL standard. See the individual function descriptions for further details.

    • A user variable referenced within a prepared statement now has its data type determined when the statement is prepared; the type persists for each subsequent execution of the statement.

    • A user variable referenced by a statement occurring within a stored procedure now has its data type determined the first time the statement is executed; the type persists for any subsequent invocation of the containing stored procedure.

    • When executing a prepared statement of the form SELECT expr1, expr2, ... FROM table ORDER BY ?, passing an integer value N for the parameter no longer causes ordering of the results by the Nth expression in the select list; the results are no longer ordered, as is expected with ORDER BY constant.

    Preparing a statement used as a prepared statement or within a stored procedure only once enhances the performance of the statement, since it negates the added cost of repeated preparation. Doing so also avoids possible multiple rollbacks of preparation structures, which has been the source of numerous issues in MySQL.

    For more information, see Section 13.5.1, “PREPARE Statement”.

  • RIGHT JOIN as LEFT JOIN handling.  As of MySQL 8.0.22, the server handles all instances of RIGHT JOIN internally as LEFT JOIN, eliminating a number of special cases in which a complete conversion was not performed at parse time.

  • Derived condition pushdown optimization.  MySQL 8.0.22 (and later) implements derived condition pushdown for queries having materialized derived tables. For a query such as SELECT * FROM (SELECT i, j FROM t1) AS dt WHERE i > constant, it is now possible in many cases to push the the outer WHERE condition down to the derived table, in this case resulting in SELECT * FROM (SELECT i, j FROM t1 WHERE i > constant) AS dt.

    Previously, if the derived table was materialized and not merged, MySQL materialized the entire table, then qualified the rows with the WHERE condition. Moving the WHERE condition into the subquery using the derived condition pushdown optimization can often reduce the number of rows must be be processed, which can decrease the time needed to execute the query.

    An outer WHERE condition can be pushed down directly to a materialized derived table when the derived table does not use any aggregate or window functions. When the derived table has a GROUP BY and does not use any window functions, the outer WHERE condition can be pushed down to the derived table as a HAVING condition. The WHERE condition can also be pushed down when the derived table uses a window function and the outer WHERE references columns used in the window function's PARTITION clause.

    Derived condition pushdown is enabled by default, as indicated by the optimizer_switch system variable's derived_condition_pushdown flag. The flag, added in MySQL 8.0.22, is set to on by default; to disable the optimization for a specific query, you can use the NO_DERIVED_CONDITION_PUSHDOWN optimizer hint (also added in MySQL 8.0.22). If the optimization is disabled due to derived_condition_pushdown being set to off, you can enable it for a given query using DERIVED_CONDITION_PUSHDOWN.

    The derived condition pushdown optimization cannot be employed for a derived table that contains a UNION or LIMIT clause. In addition, a condition that itself uses a subquery cannot be pushed down, and a WHERE condition cannot be pushed down to a derived table that is also an inner table of an outer join. For additional information and examples, see Section, “Derived Condition Pushdown Optimization”.

  • Non-locking reads on MySQL grant tables.  As of MySQL 8.0.22, to permit concurrent DML and DDL operations on MySQL grant tables, read operations that previously acquired row locks on MySQL grant tables are executed as non-locking reads.

    The operations that are now performed as non-locking reads on MySQL grant tables include:

    • SELECT statements and other read-only statements that read data from grant tables through join lists and subqueries, including SELECT ... FOR SHARE statements, using any transaction isolation level.

    • DML operations that read data from grant tables (through join lists or subqueries) but do not modify them, using any transaction isolation level.

    For additional information, see Grant Table Concurrency.

Features Deprecated in MySQL 8.0

The following features are deprecated in MySQL 8.0 and may be removed in a future series. Where alternatives are shown, applications should be updated to use them.

For applications that use features deprecated in MySQL 8.0 that have been removed in a higher MySQL series, statements may fail when replicated from a MySQL 8.0 source to a higher-series replica, or may have different effects on source and replica. To avoid such problems, applications that use features deprecated in 8.0 should be revised to avoid them and use alternatives when possible.

  • The utf8mb3 character set is deprecated. Please use utf8mb4 instead.

  • Because caching_sha2_password is the default authentication plugin in MySQL 8.0 and provides a superset of the capabilities of the sha256_password authentication plugin, sha256_password is deprecated; expect it to be removed in a future version of MySQL. MySQL accounts that authenticate using sha256_password should be migrated to use caching_sha2_password instead.

  • The validate_password plugin has been reimplemented to use the component infrastructure. The plugin form of validate_password is still available but is now deprecated; expect it to be removed in a future version of MySQL. MySQL installations that use the plugin should make the transition to using the component instead. See Section, “Transitioning to the Password Validation Component”.

  • The ENGINE clause for the ALTER TABLESPACE and DROP TABLESPACE statements is deprecated.

  • The PAD_CHAR_TO_FULL_LENGTH SQL mode is deprecated.

  • AUTO_INCREMENT support is deprecated for columns of type FLOAT and DOUBLE (and any synonyms). Consider removing the AUTO_INCREMENT attribute from such columns, or convert them to an integer type.

  • The UNSIGNED attribute is deprecated for columns of type FLOAT, DOUBLE, and DECIMAL (and any synonyms). Consider using a simple CHECK constraint instead for such columns.

  • FLOAT(M,D) and DOUBLE(M,D) syntax to specify the number of digits for columns of type FLOAT and DOUBLE (and any synonyms) is a nonstandard MySQL extension. This syntax is deprecated.

  • The ZEROFILL attribute is deprecated for numeric data types, as is the display width attribute for integer data types. Consider using an alternative means of producing the effect of these attributes. For example, applications could use the LPAD() function to zero-pad numbers up to the desired width, or they could store the formatted numbers in CHAR columns.

  • For string data types, the BINARY attribute is a nonstandard MySQL extension that is shorthand for specifying the binary (_bin) collation of the column character set (or of the table default character set if no column character set is specified). In MySQL 8.0, this nonstandard use of BINARY is ambiguous because the utf8mb4 character set has multiple _bin collations, so the BINARY attribute is deprecated; expect support for it to be removed in a future version of MySQL. Applications should be adjusted to use an explicit _bin collation instead.

    The use of BINARY to specify a data type or character set remains unchanged.

  • The nonstandard C-style &&, ||, and ! operators that are synonyms for the standard SQL AND, OR, and NOT operators, respectively, are deprecated. Applications that use the nonstandard operators should be adjusted to use the standard operators.


    Use of || is deprecated unless the PIPES_AS_CONCAT SQL mode is enabled. In that case, || signifies the SQL-standard string concatenation operator).

  • The JSON_MERGE() function is deprecated. Use JSON_MERGE_PRESERVE() instead.

  • The SQL_CALC_FOUND_ROWS query modifier and accompanying FOUND_ROWS() function are deprecated. See the FOUND_ROWS() description for information about an alternative strategy.

  • Support for TABLESPACE = innodb_file_per_table and TABLESPACE = innodb_temporary clauses with CREATE TEMPORARY TABLE is deprecated as of MySQL 8.0.13.

  • For SELECT statements, use of an INTO clause after FROM but not at the end of the SELECT is deprecated as of MySQL 8.0.20. It is preferred to place the INTO at the end of the statement.

    For UNION statements, these two variants containing INTO are deprecated as of MySQL 8.0.20:

    • In the trailing query block of a query expression, use of INTO before FROM.

    • In a parenthesized trailing block of a query expression, use of INTO, regardless of its position relative to FROM.

    See Section, “SELECT ... INTO Statement”, and Section, “UNION Clause”.

  • FLUSH HOSTS is deprecated as of MySQL 8.0.23. Instead, truncate the Performance Schema host_cache table:

    TRUNCATE TABLE performance_schema.host_cache;

    The TRUNCATE TABLE operation requires the DROP privilege for the table.

  • The mysql_upgrade client is deprecated because its capabilities for upgrading the system tables in the mysql system schema and objects in other schemas have been moved into the MySQL server. See Section 2.11.3, “What the MySQL Upgrade Process Upgrades”.

  • The --no-dd-upgrade server option is deprecated. It is superseded by the --upgrade option, which provides finer control over data dictionary and server upgrade behavior.

  • The mysql_upgrade_info file, which is created data directory and used to store the MySQL version number, is deprecated; expect it to be removed in a future version of MySQL.

  • The relay_log_info_file system variable and --master-info-file option are deprecated. Previously, these were used to specify the name of the relay log info log and source info log when relay_log_info_repository=FILE and master_info_repository=FILE were set, but those settings have been deprecated. The use of files for the relay log info log and source info log has been superseded by crash-safe replica tables, which are the default in MySQL 8.0.

  • The max_length_for_sort_data system variable is now deprecated due to optimizer changes that make it obsolete and of no effect.

  • These legacy parameters for compression of connections to the server are deprecated: The --compress client command-line option; the MYSQL_OPT_COMPRESS option for the mysql_options() C API function; the slave_compressed_protocol system variable. For information about parameters to use instead, see Section 4.2.8, “Connection Compression Control”.

  • Use of the MYSQL_PWD environment variable to specify a MySQL password is deprecated.

  • Use of VALUES() to access new row values in INSERT ... ON DUPLICATE KEY UPDATE is deprecated as of MySQL 8.0.20. Use aliases for the new row and columns, instead.

  • Because specifying ON ERROR before ON EMPTY when invoking JSON_TABLE() is counter to the SQL standard, this syntax is now deprecated in MySQL. Beginning with MySQL 8.0.20, the server prints a warning whenever you attempt to do so. When specifying both of these clauses in a single JSON_TABLE() invocation, make sure that ON EMPTY is used first.

  • Columns with index prefixes have never been supported as part of a table's partitioning key; previously, these were allowed when creating, altering, or upgrading partitioned tables but were excluded by the table's partitioning function, and no warning that this had occurred was issued by the server. This permissive behavior is now deprecated, and subject to removal in a future version of MySQL in which using any such columns in the partitioning key causes the CREATE TABLE or ALTER TABLE statement in they occur to be rejected.

    As of MySQL 8.0.21, whenever columns using index prefixes are specified as part of the partitioning key, a warning is generated for each such column. Whenever a CREATE TABLE or ALTER TABLE statement is rejected because all columns in the proposed partitioning key would have index prefixes, the resulting error now provides the exact reason for the rejection. In either instance, this includes cases in which the columns used in the partitioning function are defined implicitly as those in the table's primary key by employing an empty PARTITION BY KEY() clause.

    For more information and examples, see Column index prefixes not supported for key partitioning.

  • The InnoDB memcached plugin is deprecated as of MySQL 8.0.22; expect support for it to be removed in a future version of MySQL.

Features Removed in MySQL 8.0

The following items are obsolete and have been removed in MySQL 8.0. Where alternatives are shown, applications should be updated to use them.

For MySQL 5.7 applications that use features removed in MySQL 8.0, statements may fail when replicated from a MySQL 5.7 source to a MySQL 8.0 replica, or may have different effects on source and replica. To avoid such problems, applications that use features removed in MySQL 8.0 should be revised to avoid them and use alternatives when possible.

  • The innodb_locks_unsafe_for_binlog system variable was removed. The READ COMMITTED isolation level provides similar functionality.

  • The information_schema_stats variable, introduced in MySQL 8.0.0, was removed and replaced by information_schema_stats_expiry in MySQL 8.0.3.

    information_schema_stats_expiry defines an expiration setting for cached INFORMATION_SCHEMA table statistics. For more information, see Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”.

  • Code related to obsoleted InnoDB system tables was removed in MySQL 8.0.3. INFORMATION_SCHEMA views based on InnoDB system tables were replaced by internal system views on data dictionary tables. Affected InnoDB INFORMATION_SCHEMA views were renamed:

    Table 1.1 Renamed InnoDB Information Schema Views

    Old Name New Name

    After upgrading to MySQL 8.0.3 or later, update any scripts that reference previous InnoDB INFORMATION_SCHEMA view names.

  • The following features related to account management are removed:

    • Using GRANT to create users. Instead, use CREATE USER. Following this practice makes the NO_AUTO_CREATE_USER SQL mode immaterial for GRANT statements, so it too is removed, and an error now is written to the server log when the presence of this value for the sql_mode option in the options file prevents mysqld from starting.

    • Using GRANT to modify account properties other than privilege assignments. This includes authentication, SSL, and resource-limit properties. Instead, establish such properties at account-creation time with CREATE USER or modify them afterward with ALTER USER.

    • IDENTIFIED BY PASSWORD 'auth_string' syntax for CREATE USER and GRANT. Instead, use IDENTIFIED WITH auth_plugin AS 'auth_string' for CREATE USER and ALTER USER, where the 'auth_string' value is in a format compatible with the named plugin.

      Additionally, because IDENTIFIED BY PASSWORD syntax was removed, the log_builtin_as_identified_by_password system variable is superfluous and was removed.

    • The PASSWORD() function. Additionally, PASSWORD() removal means that SET PASSWORD ... = PASSWORD('auth_string') syntax is no longer available.

    • The old_passwords system variable.

  • The query cache was removed. Removal includes these items:


    • These system variables: query_cache_limit, query_cache_min_res_unit, query_cache_size, query_cache_type, query_cache_wlock_invalidate.

    • These status variables: Qcache_free_blocks, Qcache_free_memory, Qcache_hits, Qcache_inserts, Qcache_lowmem_prunes, Qcache_not_cached, Qcache_queries_in_cache, Qcache_total_blocks.

    • These thread states: checking privileges on cached query, checking query cache for query, invalidating query cache entries, sending cached result to client, storing result in query cache, Waiting for query cache lock.

    • The SQL_CACHE SELECT modifier.

    These deprecated query cache items remain deprecated, but have no effect; expect them to be removed in a future MySQL release:

    • The SQL_NO_CACHE SELECT modifier.

    • The ndb_cache_check_time system variable.

    The have_query_cache system variable remains deprecated, and always has a value of NO; expect it to be removed in a future MySQL release.

  • The data dictionary provides information about database objects, so the server no longer checks directory names in the data directory to find databases. Consequently, the --ignore-db-dir option and ignore_db_dirs system variables are extraneous and are removed.

  • The DDL log, also known as the metadata log, has been removed. Beginning with MySQL 8.0.3, this functionality is handled by the data dictionary innodb_ddl_log table. See Viewing DDL Logs.

  • The tx_isolation and tx_read_only system variables have been removed. Use transaction_isolation and transaction_read_only instead.

  • The sync_frm system variable has been removed because .frm files have become obsolete.

  • The secure_auth system variable and --secure-auth client option have been removed. The MYSQL_SECURE_AUTH option for the mysql_options() C API function was removed.

  • The multi_range_count system variable is removed.

  • The log_warnings system variable and --log-warnings server option have been removed. Use the log_error_verbosity system variable instead.

  • The global scope for the sql_log_bin system variable was removed. sql_log_bin has session scope only, and applications that rely on accessing @@GLOBAL.sql_log_bin should be adjusted.

  • The metadata_locks_cache_size and metadata_locks_hash_instances system variables are removed.

  • The unused date_format, datetime_format, time_format, and max_tmp_tables system variables are removed.

  • These deprecated compatibility SQL modes are removed: DB2, MAXDB, MSSQL, MYSQL323, MYSQL40, ORACLE, POSTGRESQL, NO_FIELD_OPTIONS, NO_KEY_OPTIONS, NO_TABLE_OPTIONS. They can no longer be assigned to the sql_mode system variable or used as permitted values for the mysqldump --compatible option.

    Removal of MAXDB means that the TIMESTAMP data type for CREATE TABLE or ALTER TABLE is treated as TIMESTAMP, and is no longer treated as DATETIME.

  • The deprecated ASC or DESC qualifiers for GROUP BY clauses are removed. Queries that previously relied on GROUP BY sorting may produce results that differ from previous MySQL versions. To produce a given sort order, provide an ORDER BY clause.

  • The EXTENDED and PARTITIONS keywords for the EXPLAIN statement have been removed. These keywords are unnecessary because their effect is always enabled.

  • These encryption-related items are removed:

    • The ENCODE() and DECODE() functions.

    • The ENCRYPT() function.

    • The DES_ENCRYPT(), and DES_DECRYPT() functions, the --des-key-file option, the have_crypt system variable, the DES_KEY_FILE option for the FLUSH statement, and the HAVE_CRYPT CMake option.

    In place of the removed encryption functions: For ENCRYPT(), consider using SHA2() instead for one-way hashing. For the others, consider using AES_ENCRYPT() and AES_DECRYPT() instead.

  • In MySQL 5.7, several spatial functions available under multiple names were deprecated to move in the direction of making the spatial function namespace more consistent, the goal being that each spatial function name begin with ST_ if it performs an exact operation, or with MBR if it performs an operation based on minimum bounding rectangles. In MySQL 8.0, the deprecated functions are removed to leave only the corresponding ST_ and MBR functions:

    • These functions are removed in favor of the MBR names: Contains(), Disjoint(), Equals(), Intersects(), Overlaps(), Within().

    • These functions are removed in favor of the ST_ names: Area(), AsBinary(), AsText(), AsWKB(), AsWKT(), Buffer(), Centroid(), ConvexHull(), Crosses(), Dimension(), Distance(), EndPoint(), Envelope(), ExteriorRing(), GeomCollFromText(), GeomCollFromWKB(), GeomFromText(), GeomFromWKB(), GeometryCollectionFromText(), GeometryCollectionFromWKB(), GeometryFromText(), GeometryFromWKB(), GeometryN(), GeometryType(), InteriorRingN(), IsClosed(), IsEmpty(), IsSimple(), LineFromText(), LineFromWKB(), LineStringFromText(), LineStringFromWKB(), MLineFromText(), MLineFromWKB(), MPointFromText(), MPointFromWKB(), MPolyFromText(), MPolyFromWKB(), MultiLineStringFromText(), MultiLineStringFromWKB(), MultiPointFromText(), MultiPointFromWKB(), MultiPolygonFromText(), MultiPolygonFromWKB(), NumGeometries(), NumInteriorRings(), NumPoints(), PointFromText(), PointFromWKB(), PointN(), PolyFromText(), PolyFromWKB(), PolygonFromText(), PolygonFromWKB(), SRID(), StartPoint(), Touches(), X(), Y().

    • GLength() is removed in favor of ST_Length().

  • The functions described in Section 12.17.4, “Functions That Create Geometry Values from WKB Values” previously accepted either WKB strings or geometry arguments. Geometry arguments are no longer permitted and produce an error. See that section for guidelines for migrating queries away from using geometry arguments.

  • The parser no longer treats \N as a synonym for NULL in SQL statements. Use NULL instead.

    This change does not affect text file import or export operations performed with LOAD DATA or SELECT ... INTO OUTFILE, for which NULL continues to be represented by \N. See Section 13.2.7, “LOAD DATA Statement”.

  • PROCEDURE ANALYSE() syntax is removed.

  • The client-side --ssl and --ssl-verify-server-cert options have been removed. Use --ssl-mode=REQUIRED instead of --ssl=1 or --enable-ssl. Use --ssl-mode=DISABLED instead of --ssl=0, --skip-ssl, or --disable-ssl. Use --ssl-mode=VERIFY_IDENTITY instead of --ssl-verify-server-cert options. (The server-side --ssl option remains unchanged.)

    For the C API, MYSQL_OPT_SSL_ENFORCE and MYSQL_OPT_SSL_VERIFY_SERVER_CERT options for mysql_options() correspond to the client-side --ssl and --ssl-verify-server-cert options and are removed. Use MYSQL_OPT_SSL_MODE with an option value of SSL_MODE_REQUIRED or SSL_MODE_VERIFY_IDENTITY instead.

  • The --temp-pool server option was removed.

  • The ignore_builtin_innodb system variable is removed.

  • The server no longer performs conversion of pre-MySQL 5.1 database names containing special characters to 5.1 format with the addition of a #mysql50# prefix. Because these conversions are no longer performed, the --fix-db-names and --fix-table-names options for mysqlcheck, the UPGRADE DATA DIRECTORY NAME clause for the ALTER DATABASE statement, and the Com_alter_db_upgrade status variable are removed.

    Upgrades are supported only from one major version to another (for example, 5.0 to 5.1, or 5.1 to 5.5), so there should be little remaining need for conversion of older 5.0 database names to current versions of MySQL. As a workaround, upgrade a MySQL 5.0 installation to MySQL 5.1 before upgrading to a more recent release.

  • The mysql_install_db program has been removed from MySQL distributions. Data directory initialization should be performed by invoking mysqld with the --initialize or --initialize-insecure option instead. In addition, the --bootstrap option for mysqld that was used by mysql_install_db was removed, and the INSTALL_SCRIPTDIR CMake option that controlled the installation location for mysql_install_db was removed.

  • The generic partitioning handler was removed from the MySQL server. In order to support partitioning of a given table, the storage engine used for the table must now provide its own (native) partitioning handler. The --partition and --skip-partition options are removed from the MySQL Server, and partitioning-related entries are no longer shown in the output of SHOW PLUGINS or in the INFORMATION_SCHEMA.PLUGINS table.

    Two MySQL storage engines currently provide native partitioning support: InnoDB and NDB. Of these, only InnoDB is supported in MySQL 8.0. Any attempt to create partitioned tables in MySQL 8.0 using any other storage engine fails.

    Ramifications for upgrades.  The direct upgrade of a partitioned table using a storage engine other than InnoDB (such as MyISAM) from MySQL 5.7 (or earlier) to MySQL 8.0 is not supported. There are two options for handling such a table:

    At least one of the two operations just listed must be performed for each partitioned non-InnoDB table prior to upgrading the server to MySQL 8.0. Otherwise, such a table cannot be used following the upgrade.

    Due to the fact that table creation statements that would result in a partitioned table using a storage engine without partitioning support now fail with an error (ER_CHECK_NOT_IMPLEMENTED), you must make sure that any statements in a dump file (such as that written by mysqldump) from an older version of MySQL that you wish to import into a MySQL 8.0 server that create partitioned tables do not also specify a storage engine such as MyISAM that has no native partitioning handler. You can do this by performing either of the following:

    • Remove any references to partitioning from CREATE TABLE statements that use a value for the STORAGE ENGINE option other than InnoDB.

    • Specifying the storage engine as InnoDB, or allow InnoDB to be used as the table's storage engine by default.

    For more information, see Section 24.6.2, “Partitioning Limitations Relating to Storage Engines”.

  • System and status variable information is no longer maintained in the INFORMATION_SCHEMA. These tables are removed: GLOBAL_VARIABLES, SESSION_VARIABLES, GLOBAL_STATUS, SESSION_STATUS. Use the corresponding Performance Schema tables instead. See Section 27.12.14, “Performance Schema System Variable Tables”, and Section 27.12.15, “Performance Schema Status Variable Tables”. In addition, the show_compatibility_56 system variable was removed. It was used in the transition period during which system and status variable information in INFORMATION_SCHEMA tables was moved to Performance Schema tables, and is no longer needed. These status variables are removed: Slave_heartbeat_period, Slave_last_heartbeat, Slave_received_heartbeats, Slave_retried_transactions, Slave_running. The information they provided is available in Performance Schema tables; see Migrating to Performance Schema System and Status Variable Tables.

  • The Performance Schema setup_timers table was removed, as was the TICK row in the performance_timers table.

  • The libmysqld embedded server library is removed, along with:


    • The mysql_config --libmysqld-libs, --embedded-libs, and --embedded options


    • The (undocumented) mysql --server-arg option

    • The mysqltest --embedded-server, --server-arg, and --server-file options

    • The mysqltest_embedded and mysql_client_test_embedded test programs

  • The mysql_plugin utility was removed. Alternatives include loading plugins at server startup using the --plugin-load or --plugin-load-add option, or at runtime using the INSTALL PLUGIN statement.

  • The resolveip utility is removed. nslookup, host, or dig can be used instead.

  • The resolve_stack_dump utility is removed. Stack traces from official MySQL builds are always symbolized, so there is no need to use resolve_stack_dump.

  • The following server error codes are not used and have been removed. Applications that test specifically for any of these errors should be updated.

  • The deprecated INFORMATION_SCHEMA INNODB_LOCKS and INNODB_LOCK_WAITS tables are removed. Use the Performance Schema data_locks and data_lock_waits tables instead.


    In MySQL 5.7, the LOCK_TABLE column in the INNODB_LOCKS table and the locked_table column in the sys schema innodb_lock_waits and x$innodb_lock_waits views contain combined schema/table name values. In MySQL 8.0, the data_locks table and the sys schema views contain separate schema name and table name columns. See Section, “The innodb_lock_waits and x$innodb_lock_waits Views”.

  • InnoDB no longer supports compressed temporary tables. When innodb_strict_mode is enabled (the default), CREATE TEMPORARY TABLE returns an error if ROW_FORMAT=COMPRESSED or KEY_BLOCK_SIZE is specified. If innodb_strict_mode is disabled, warnings are issued and the temporary table is created using a non-compressed row format.

  • InnoDB no longer creates .isl files (InnoDB Symbolic Link files) when creating tablespace data files outside of the MySQL data directory. The innodb_directories option now supports locating tablespace files created outside of the data directory.

    With this change, moving a remote tablespace while the server is offline by manually modifying an .isl file is no longer supported. Moving remote tablespace files is now supported by the innodb_directories option. See Section, “Moving Tablespace Files While the Server is Offline”.

  • The following InnoDB file format variables were removed:

    • innodb_file_format

    • innodb_file_format_check

    • innodb_file_format_max

    • innodb_large_prefix

    File format variables were necessary for creating tables compatible with earlier versions of InnoDB in MySQL 5.1. Now that MySQL 5.1 has reached the end of its product lifecycle, these options are no longer required.

    The FILE_FORMAT column was removed from the INNODB_TABLES and INNODB_TABLESPACES Information Schema tables.

  • The innodb_support_xa system variable, which enables support for two-phase commit in XA transactions, was removed. InnoDB support for two-phase commit in XA transactions is always enabled.

  • Support for DTrace was removed.

  • The JSON_APPEND() function was removed. Use JSON_ARRAY_APPEND() instead.

  • Support for placing table partitions in shared InnoDB tablespaces was removed in MySQL 8.0.13. Shared tablespaces include the InnoDB system tablespace and general tablespaces. For information about identifying partitions in shared tablespaces and moving them to file-per-table tablespaces, see Section 2.11.5, “Preparing Your Installation for Upgrade”.

  • Support for setting user variables in statements other than SET was deprecated in MySQL 8.0.13. This functionality is subject to removal in MySQL 9.0.

  • The --ndb perror option was removed. Use the ndb_perror utility instead.

  • The innodb_undo_logs variable was removed. The innodb_rollback_segments variables performs the same function and should be used instead.

  • The Innodb_available_undo_logs status variable was removed. The number of available rollback segments per tablespace may be retrieved using SHOW VARIABLES LIKE 'innodb_rollback_segments';

  • As of MySQL 8.0.14, the previously deprecated innodb_undo_tablespaces variable is no longer configurable. For more information, see Section, “Undo Tablespaces”.

  • Support for the ALTER TABLE ... UPGRADE PARTITIONING statement has been removed.

  • As of MySQL 8.0.16, support for the internal_tmp_disk_storage_engine system variable has been removed; internal temporary tables on disk now always use the InnoDB storage engine. See Storage Engine for On-Disk Internal Temporary Tables,for more information.

  • The DISABLE_SHARED CMake option was unused and has been removed.

1.4 Server and Status Variables and Options Added, Deprecated, or Removed in MySQL 8.0

This section lists server variables, status variables, and options that were added for the first time, have been deprecated, or have been removed in MySQL 8.0.

Options and Variables Introduced in MySQL 8.0

The following system variables, status variables, and server options have been added in MySQL 8.0.

Options and Variables Deprecated in MySQL 8.0

The following system variables, status variables, and options have been deprecated in MySQL 8.0.

  • Compression: Whether client connection uses compression in client/server protocol. Deprecated as of MySQL 8.0.18.

  • expire_logs_days: Purge binary logs after this many days. Deprecated as of MySQL 8.0.3.

  • group_replication_ip_whitelist: List of hosts permitted to connect to group. Deprecated as of MySQL 8.0.22.

  • innodb_undo_tablespaces: Number of tablespace files that rollback segments are divided between. Deprecated as of MySQL 8.0.4.

  • log_bin_use_v1_row_events: Whether server is using version 1 binary log row events. Deprecated as of MySQL 8.0.18.

  • log_syslog: Whether to write error log to syslog. Deprecated as of MySQL 8.0.2.

  • master-info-file: Location and name of file that remembers source and where I/O replication thread is in source's binary log. Deprecated as of MySQL 8.0.18.

  • master_info_repository: Whether to write connection metadata repository, containing source information and replication I/O thread location in source's binary log, to file or table. Deprecated as of MySQL 8.0.23.

  • max_length_for_sort_data: Max number of bytes in sorted records. Deprecated as of MySQL 8.0.20.

  • no-dd-upgrade: Prevent automatic upgrade of data dictionary tables at startup. Deprecated as of MySQL 8.0.16.

  • relay_log_info_file: File name for applier metadata repository in which replica records information about relay logs. Deprecated as of MySQL 8.0.18.

  • relay_log_info_repository: Whether to write location of replication SQL thread in relay logs to file or table. Deprecated as of MySQL 8.0.23.

  • slave_compressed_protocol: Use compression of source/replica protocol. Deprecated as of MySQL 8.0.18.

  • slave_rows_search_algorithms: Determines search algorithms used for replica update batching. Any 2 or 3 from this list: INDEX_SEARCH, TABLE_SCAN, HASH_SCAN. Deprecated as of MySQL 8.0.18.

  • symbolic-links: Permit symbolic links for MyISAM tables. Deprecated as of MySQL 8.0.2.

Options and Variables Removed in MySQL 8.0

The following system variables, status variables, and options have been removed in MySQL 8.0.

  • Com_alter_db_upgrade: Count of ALTER DATABASE ... UPGRADE DATA DIRECTORY NAME statements. Removed in MySQL 8.0.0.

  • Innodb_available_undo_logs: Total number of InnoDB rollback segments; different from innodb_rollback_segments, which displays number of active rollback segments. Removed in MySQL 8.0.2.

  • Qcache_free_blocks: Number of free memory blocks in query cache. Removed in MySQL 8.0.3.

  • Qcache_free_memory: Amount of free memory for query cache. Removed in MySQL 8.0.3.

  • Qcache_hits: Number of query cache hits. Removed in MySQL 8.0.3.

  • Qcache_inserts: Number of query cache inserts. Removed in MySQL 8.0.3.

  • Qcache_lowmem_prunes: Number of queries which were deleted from query cache due to lack of free memory in cache. Removed in MySQL 8.0.3.

  • Qcache_not_cached: Number of noncached queries (not cacheable, or not cached due to query_cache_type setting). Removed in MySQL 8.0.3.

  • Qcache_queries_in_cache: Number of queries registered in query cache. Removed in MySQL 8.0.3.

  • Qcache_total_blocks: Total number of blocks in query cache. Removed in MySQL 8.0.3.

  • Slave_heartbeat_period: Replica's replication heartbeat interval, in seconds. Removed in MySQL 8.0.1.

  • Slave_last_heartbeat: Shows when latest heartbeat signal was received, in TIMESTAMP format. Removed in MySQL 8.0.1.

  • Slave_received_heartbeats: Number of heartbeats received by replica since previous reset. Removed in MySQL 8.0.1.

  • Slave_retried_transactions: Total number of times since startup that replication SQL thread has retried transactions. Removed in MySQL 8.0.1.

  • Slave_running: State of this server as replica (replication I/O thread status). Removed in MySQL 8.0.1.

  • bootstrap: Used by mysql installation scripts. Removed in MySQL 8.0.0.

  • date_format: DATE format (unused). Removed in MySQL 8.0.3.

  • datetime_format: DATETIME/TIMESTAMP format (unused). Removed in MySQL 8.0.3.

  • des-key-file: Load keys for des_encrypt() and des_encrypt from given file. Removed in MySQL 8.0.3.

  • group_replication_allow_local_disjoint_gtids_join: Allow current server to join group even if it has transactions not present in group. Removed in MySQL 8.0.4.

  • have_crypt: Availability of crypt() system call. Removed in MySQL 8.0.3.

  • ignore-db-dir: Treat directory as nondatabase directory. Removed in MySQL 8.0.0.

  • ignore_builtin_innodb: Ignore built-in InnoDB. Removed in MySQL 8.0.3.

  • ignore_db_dirs: Directories treated as nondatabase directories. Removed in MySQL 8.0.0.

  • innodb_checksums: Enable InnoDB checksums validation. Removed in MySQL 8.0.0.

  • innodb_disable_resize_buffer_pool_debug: Disables resizing of InnoDB buffer pool. Removed in MySQL 8.0.0.

  • innodb_file_format: Format for new InnoDB tables. Removed in MySQL 8.0.0.

  • innodb_file_format_check: Whether InnoDB performs file format compatibility checking. Removed in MySQL 8.0.0.

  • innodb_file_format_max: File format tag in shared tablespace. Removed in MySQL 8.0.0.

  • innodb_large_prefix: Enables longer keys for column prefix indexes. Removed in MySQL 8.0.0.

  • innodb_locks_unsafe_for_binlog: Force InnoDB not to use next-key locking. Instead use only row-level locking. Removed in MySQL 8.0.0.

  • innodb_scan_directories: Defines directories to scan for tablespace files during InnoDB recovery. Removed in MySQL 8.0.4.

  • innodb_stats_sample_pages: Number of index pages to sample for index distribution statistics. Removed in MySQL 8.0.0.

  • innodb_support_xa: Enable InnoDB support for XA two-phase commit. Removed in MySQL 8.0.0.

  • innodb_undo_logs: Number of undo logs (rollback segments) used by InnoDB; alias for innodb_rollback_segments. Removed in MySQL 8.0.2.

  • internal_tmp_disk_storage_engine: Storage engine for internal temporary tables. Removed in MySQL 8.0.16.

  • log-warnings: Write some noncritical warnings to log file. Removed in MySQL 8.0.3.

  • log_builtin_as_identified_by_password: Whether to log CREATE/ALTER USER, GRANT in backward-compatible fashion. Removed in MySQL 8.0.11.

  • log_error_filter_rules: Filter rules for error logging. Removed in MySQL 8.0.4.

  • log_syslog: Whether to write error log to syslog. Removed in MySQL 8.0.13.

  • log_syslog_facility: Facility for syslog messages. Removed in MySQL 8.0.13.

  • log_syslog_include_pid: Whether to include server PID in syslog messages. Removed in MySQL 8.0.13.

  • log_syslog_tag: Tag for server identifier in syslog messages. Removed in MySQL 8.0.13.

  • max_tmp_tables: Unused. Removed in MySQL 8.0.3.

  • metadata_locks_cache_size: Size of metadata locks cache. Removed in MySQL 8.0.13.

  • metadata_locks_hash_instances: Number of metadata lock hashes. Removed in MySQL 8.0.13.

  • multi_range_count: Maximum number of ranges to send to table handler at once during range selects. Removed in MySQL 8.0.3.

  • old_passwords: Selects password hashing method for PASSWORD(). Removed in MySQL 8.0.11.

  • partition: Enable (or disable) partitioning support. Removed in MySQL 8.0.0.

  • query_cache_limit: Do not cache results that are bigger than this. Removed in MySQL 8.0.3.

  • query_cache_min_res_unit: Minimal size of unit in which space for results is allocated (last unit is trimmed after writing all result data). Removed in MySQL 8.0.3.

  • query_cache_size: Memory allocated to store results from old queries. Removed in MySQL 8.0.3.

  • query_cache_type: Query cache type. Removed in MySQL 8.0.3.

  • query_cache_wlock_invalidate: Invalidate queries in query cache on LOCK for write. Removed in MySQL 8.0.3.

  • secure_auth: Disallow authentication for accounts that have old (pre-4.1) passwords. Removed in MySQL 8.0.3.

  • show_compatibility_56: Compatibility for SHOW STATUS/VARIABLES. Removed in MySQL 8.0.1.

  • skip-partition: Do not enable user-defined partitioning. Removed in MySQL 8.0.0.

  • sync_frm: Sync .frm to disk on create. Enabled by default. Removed in MySQL 8.0.0.

  • temp-pool: Using this option causes most temporary files created to use small set of names, rather than unique name for each new file. Removed in MySQL 8.0.1.

  • time_format: TIME format (unused). Removed in MySQL 8.0.3.

  • tx_isolation: Default transaction isolation level. Removed in MySQL 8.0.3.

  • tx_read_only: Default transaction access mode. Removed in MySQL 8.0.3.

1.5 MySQL Information Sources

This section lists sources of additional information that you may find helpful, such as MySQL websites, mailing lists, user forums, and Internet Relay Chat.

MySQL Websites

The primary website for MySQL documentation is Online and downloadable documentation formats are available for the MySQL Reference Manual, MySQL Connectors, and more.

The MySQL developers provide information about new and upcoming features as the MySQL Server Blog.

MySQL Community Support at the MySQL Forums

The forums at are an important community resource. Many forums are available, grouped into these general categories:

  • Migration

  • MySQL Usage

  • MySQL Connectors

  • Programming Languages

  • Tools

  • 3rd-Party Applications

  • Storage Engines

  • MySQL Technology

  • SQL Standards

  • Business

MySQL Enterprise

Oracle offers technical support in the form of MySQL Enterprise. For organizations that rely on the MySQL DBMS for business-critical production applications, MySQL Enterprise is a commercial subscription offering which includes:

  • MySQL Enterprise Server

  • MySQL Enterprise Monitor

  • Monthly Rapid Updates and Quarterly Service Packs

  • MySQL Knowledge Base

  • 24x7 Technical and Consultative Support

MySQL Enterprise is available in multiple tiers, giving you the flexibility to choose the level of service that best matches your needs. For more information, see MySQL Enterprise.

1.6 How to Report Bugs or Problems

Before posting a bug report about a problem, please try to verify that it is a bug and that it has not been reported already:

  • Start by searching the MySQL online manual at We try to keep the manual up to date by updating it frequently with solutions to newly found problems. In addition, the release notes accompanying the manual can be particularly useful since it is quite possible that a newer version contains a solution to your problem. The release notes are available at the location just given for the manual.

  • If you get a parse error for an SQL statement, please check your syntax closely. If you cannot find something wrong with it, it is extremely likely that your current version of MySQL Server doesn't support the syntax you are using. If you are using the current version and the manual doesn't cover the syntax that you are using, MySQL Server doesn't support your statement.

    If the manual covers the syntax you are using, but you have an older version of MySQL Server, you should check the MySQL change history to see when the syntax was implemented. In this case, you have the option of upgrading to a newer version of MySQL Server.

  • For solutions to some common problems, see Section B.3, “Problems and Common Errors”.

  • Search the bugs database at to see whether the bug has been reported and fixed.

  • You can also use to search all the Web pages (including the manual) that are located at the MySQL website.

If you cannot find an answer in the manual, the bugs database, or the mailing list archives, check with your local MySQL expert. If you still cannot find an answer to your question, please use the following guidelines for reporting the bug.

The normal way to report bugs is to visit, which is the address for our bugs database. This database is public and can be browsed and searched by anyone. If you log in to the system, you can enter new reports.

Bugs posted in the bugs database at that are corrected for a given release are noted in the release notes.

If you find a security bug in MySQL Server, please let us know immediately by sending an email message to . Exception: Support customers should report all problems, including security bugs, to Oracle Support at

To discuss problems with other users, you can use the MySQL Community Slack.

Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug in the next release. This section helps you write your report correctly so that you do not waste your time doing things that may not help us much or at all. Please read this section carefully and make sure that all the information described here is included in your report.

Preferably, you should test the problem using the latest production or development version of MySQL Server before posting. Anyone should be able to repeat the bug by just using mysql test < script_file on your test case or by running the shell or Perl script that you include in the bug report. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of everything you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See Section 5.9, “Debugging MySQL”.

Remember that it is possible for us to respond to a report containing too much information, but not to one containing too little. People often omit facts because they think they know the cause of a problem and assume that some details do not matter. A good principle to follow is that if you are in doubt about stating something, state it. It is faster and less troublesome to write a couple more lines in your report than to wait longer for the answer if we must ask you to provide information that was missing from the initial report.

The most common errors made in bug reports are (a) not including the version number of the MySQL distribution that you use, and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and version number). These are highly relevant pieces of information, and in 99 cases out of 100, the bug report is useless without them. Very often we get questions like, Why doesn't this work for me? Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed in newer MySQL versions. Errors often are platform-dependent. In such cases, it is next to impossible for us to fix anything without knowing the operating system and the version number of the platform.

If you compiled MySQL from source, remember also to provide information about your compiler if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler you used. Note that every compiling problem should be regarded as a bug and reported accordingly.

If a program produces an error message, it is very important to include the message in your report. If we try to search for something from the archives, it is better that the error message reported exactly matches the one that the program produces. (Even the lettercase should be observed.) It is best to copy and paste the entire error message into your report. You should never try to reproduce the message from memory.

If you have a problem with Connector/ODBC (MyODBC), please try to generate a trace file and send it with your report. See How to Report Connector/ODBC Problems or Bugs.

If your report includes long query output lines from test cases that you run with the mysql command-line tool, you can make the output more readable by using the --vertical option or the \G statement terminator. The EXPLAIN SELECT example later in this section demonstrates the use of \G.

Please include the following information in your report:

  • The version number of the MySQL distribution you are using (for example, MySQL 5.7.10). You can find out which version you are running by executing mysqladmin version. The mysqladmin program can be found in the bin directory under your MySQL installation directory.

  • The manufacturer and model of the machine on which you experience the problem.

  • The operating system name and version. If you work with Windows, you can usually get the name and version number by double-clicking your My Computer icon and pulling down the Help/About Windows menu. For most Unix-like operating systems, you can get this information by executing the command uname -a.

  • Sometimes the amount of memory (real and virtual) is relevant. If in doubt, include these values.

  • The contents of the docs/INFO_BIN file from your MySQL installation. This file contains information about how MySQL was configured and compiled.

  • If you are using a source distribution of the MySQL software, include the name and version number of the compiler that you used. If you have a binary distribution, include the distribution name.

  • If the problem occurs during compilation, include the exact error messages and also a few lines of context around the offending code in the file where the error occurs.

  • If mysqld died, you should also report the statement that caused mysqld to unexpectedly exit. You can usually get this information by running mysqld with query logging enabled, and then looking in the log after mysqld exits. See Section 5.9, “Debugging MySQL”.

  • If a database table is related to the problem, include the output from the SHOW CREATE TABLE db_name.tbl_name statement in the bug report. This is a very easy way to get the definition of any table in a database. The information helps us create a situation matching the one that you have experienced.

  • The SQL mode in effect when the problem occurred can be significant, so please report the value of the sql_mode system variable. For stored procedure, stored function, and trigger objects, the relevant sql_mode value is the one in effect when the object was created. For a stored procedure or function, the SHOW CREATE PROCEDURE or SHOW CREATE FUNCTION statement shows the relevant SQL mode, or you can query INFORMATION_SCHEMA for the information:


    For triggers, you can use this statement:

  • For performance-related bugs or problems with SELECT statements, you should always include the output of EXPLAIN SELECT ..., and at least the number of rows that the SELECT statement produces. You should also include the output from SHOW CREATE TABLE tbl_name for each table that is involved. The more information you provide about your situation, the more likely it is that someone can help you.

    The following is an example of a very good bug report. The statements are run using the mysql command-line tool. Note the use of the \G statement terminator for statements that would otherwise provide very long output lines that are difficult to read.

    mysql> SHOW VARIABLES;
    mysql> SHOW COLUMNS FROM ...\G
           <output from SHOW COLUMNS>
    mysql> EXPLAIN SELECT ...\G
           <output from EXPLAIN>
    mysql> FLUSH STATUS;
    mysql> SELECT ...;
           <A short version of the output from SELECT,
           including the time taken to run the query>
    mysql> SHOW STATUS;
           <output from SHOW STATUS>
  • If a bug or problem occurs while running mysqld, try to provide an input script that reproduces the anomaly. This script should include any necessary source files. The more closely the script can reproduce your situation, the better. If you can make a reproducible test case, you should upload it to be attached to the bug report.

    If you cannot provide a script, you should at least include the output from mysqladmin variables extended-status processlist in your report to provide some information on how your system is performing.

  • If you cannot produce a test case with only a few rows, or if the test table is too big to be included in the bug report (more than 10 rows), you should dump your tables using mysqldump and create a README file that describes your problem. Create a compressed archive of your files using tar and gzip or zip. After you initiate a bug report for our bugs database at, click the Files tab in the bug report for instructions on uploading the archive to the bugs database.

  • If you believe that the MySQL server produces a strange result from a statement, include not only the result, but also your opinion of what the result should be, and an explanation describing the basis for your opinion.

  • When you provide an example of the problem, it is better to use the table names, variable names, and so forth that exist in your actual situation than to come up with new names. The problem could be related to the name of a table or variable. These cases are rare, perhaps, but it is better to be safe than sorry. After all, it should be easier for you to provide an example that uses your actual situation, and it is by all means better for us. If you have data that you do not want to be visible to others in the bug report, you can upload it using the Files tab as previously described. If the information is really top secret and you do not want to show it even to us, go ahead and provide an example using other names, but please regard this as the last choice.

  • Include all the options given to the relevant programs, if possible. For example, indicate the options that you use when you start the mysqld server, as well as the options that you use to run any MySQL client programs. The options to programs such as mysqld and mysql, and to the configure script, are often key to resolving problems and are very relevant. It is never a bad idea to include them. If your problem involves a program written in a language such as Perl or PHP, please include the language processor's version number, as well as the version for any modules that the program uses. For example, if you have a Perl script that uses the DBI and DBD::mysql modules, include the version numbers for Perl, DBI, and DBD::mysql.

  • If your question is related to the privilege system, please include the output of mysqladmin reload, and all the error messages you get when trying to connect. When you test your privileges, you should execute mysqladmin reload version and try to connect with the program that gives you trouble.

  • If you have a patch for a bug, do include it. But do not assume that the patch is all we need, or that we can use it, if you do not provide some necessary information such as test cases showing the bug that your patch fixes. We might find problems with your patch or we might not understand it at all. If so, we cannot use it.

    If we cannot verify the exact purpose of the patch, we will not use it. Test cases help us here. Show that the patch handles all the situations that may occur. If we find a borderline case (even a rare one) where the patch will not work, it may be useless.

  • Guesses about what the bug is, why it occurs, or what it depends on are usually wrong. Even the MySQL team cannot guess such things without first using a debugger to determine the real cause of a bug.

  • Indicate in your bug report that you have checked the reference manual and mail archive so that others know you have tried to solve the problem yourself.

  • If your data appears corrupt or you get errors when you access a particular table, first check your tables with CHECK TABLE. If that statement reports any errors:

    • The InnoDB crash recovery mechanism handles cleanup when the server is restarted after being killed, so in typical operation there is no need to repair tables. If you encounter an error with InnoDB tables, restart the server and see whether the problem persists, or whether the error affected only cached data in memory. If data is corrupted on disk, consider restarting with the innodb_force_recovery option enabled so that you can dump the affected tables.

    • For non-transactional tables, try to repair them with REPAIR TABLE or with myisamchk. See Chapter 5, MySQL Server Administration.

    If you are running Windows, please verify the value of lower_case_table_names using the SHOW VARIABLES LIKE 'lower_case_table_names' statement. This variable affects how the server handles lettercase of database and table names. Its effect for a given value should be as described in Section 9.2.3, “Identifier Case Sensitivity”.

  • If you often get corrupted tables, you should try to find out when and why this happens. In this case, the error log in the MySQL data directory may contain some information about what happened. (This is the file with the .err suffix in the name.) See Section 5.4.2, “The Error Log”. Please include any relevant information from this file in your bug report. Normally mysqld should never corrupt a table if nothing killed it in the middle of an update. If you can find the cause of mysqld dying, it is much easier for us to provide you with a fix for the problem. See Section B.3.1, “How to Determine What Is Causing a Problem”.

  • If possible, download and install the most recent version of MySQL Server and check whether it solves your problem. All versions of the MySQL software are thoroughly tested and should work without problems. We believe in making everything as backward-compatible as possible, and you should be able to switch MySQL versions without difficulty. See Section 2.1.2, “Which MySQL Version and Distribution to Install”.

1.7 MySQL Standards Compliance

This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you can find out what they are and how to use them. You can also find information about functionality missing from MySQL Server, and how to work around some of the differences.

The SQL standard has been evolving since 1986 and several versions exist. In this manual, SQL-92 refers to the standard released in 1992. SQL:1999, SQL:2003, SQL:2008, and SQL:2011 refer to the versions of the standard released in the corresponding years, with the last being the most recent version. We use the phrase the SQL standard or standard SQL to mean the current version of the SQL Standard at any time.

One of our main goals with the product is to continue to work toward compliance with the SQL standard, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a large segment of our user base. The HANDLER interface is an example of this strategy. See Section 13.2.4, “HANDLER Statement”.

We continue to support transactional and nontransactional databases to satisfy both mission-critical 24/7 usage and heavy Web or logging usage.

MySQL Server was originally designed to work with medium-sized databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-sized databases.

We are not targeting real-time support, although MySQL replication capabilities offer significant functionality.

MySQL supports ODBC levels 0 to 3.51.

MySQL supports high-availability database clustering using the NDBCLUSTER storage engine. See Chapter 23, MySQL NDB Cluster 8.0.

We implement XML functionality which supports most of the W3C XPath standard. See Section 12.12, “XML Functions”.

MySQL supports a native JSON data type as defined by RFC 7159, and based on the ECMAScript standard (ECMA-262). See Section 11.5, “The JSON Data Type”. MySQL also implements a subset of the SQL/JSON functions specified by a pre-publication draft of the SQL:2016 standard; see Section 12.18, “JSON Functions”, for more information.

Selecting SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differently for different clients, depending on the value of the sql_mode system variable. DBAs can set the global SQL mode to match site server operating requirements, and each application can set its session SQL mode to its own requirements.

Modes affect the SQL syntax MySQL supports and the data validation checks it performs. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

For more information on setting the SQL mode, see Section 5.1.11, “Server SQL Modes”.

Running MySQL in ANSI Mode

To run MySQL Server in ANSI mode, start mysqld with the --ansi option. Running the server in ANSI mode is the same as starting it with the following options:

--transaction-isolation=SERIALIZABLE --sql-mode=ANSI

To achieve the same effect at runtime, execute these two statements:

SET GLOBAL sql_mode = 'ANSI';

You can see that setting the sql_mode system variable to 'ANSI' enables all SQL mode options that are relevant for ANSI mode as follows:

mysql> SET GLOBAL sql_mode='ANSI';
mysql> SELECT @@GLOBAL.sql_mode;

Running the server in ANSI mode with --ansi is not quite the same as setting the SQL mode to 'ANSI' because the --ansi option also sets the transaction isolation level.

See Section 5.1.7, “Server Command Options”.

1.7.1 MySQL Extensions to Standard SQL

MySQL Server supports some extensions that you are not likely to find in other SQL DBMSs. Be warned that if you use them, your code is most likely not portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the following form:

/*! MySQL-specific code */

In this case, MySQL Server parses and executes the code within the comment as it would any other SQL statement, but other SQL servers should ignore the extensions. For example, MySQL Server recognizes the STRAIGHT_JOIN keyword in the following statement, but other servers should not:

SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...

If you add a version number after the ! character, the syntax within the comment is executed only if the MySQL version is greater than or equal to the specified version number. The KEY_BLOCK_SIZE clause in the following comment is executed only by servers from MySQL 5.1.10 or higher:

CREATE TABLE t1(a INT, KEY (a)) /*!50110 KEY_BLOCK_SIZE=1024 */;

The following descriptions list MySQL extensions, organized by category.

1.7.2 MySQL Differences from Standard SQL

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but MySQL Server performs operations differently in some cases: SELECT INTO TABLE Differences

MySQL Server doesn't support the SELECT ... INTO TABLE Sybase SQL extension. Instead, MySQL Server supports the INSERT INTO ... SELECT standard SQL syntax, which is basically the same thing. See Section, “INSERT ... SELECT Statement”. For example:

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

Alternatively, you can use SELECT ... INTO OUTFILE or CREATE TABLE ... SELECT.

You can use SELECT ... INTO with user-defined variables. The same syntax can also be used inside stored routines using cursors and local variables. See Section, “SELECT ... INTO Statement”. UPDATE Differences

If you access a column from the table to be updated in an expression, UPDATE uses the current value of the column. The second assignment in the following statement sets col2 to the current (updated) col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This behavior differs from standard SQL.

UPDATE t1 SET col1 = col1 + 1, col2 = col1; FOREIGN KEY Constraint Differences

The MySQL implementation of foreign key constraints differs from the SQL standard in the following key respects:

  • If there are several rows in the parent table with the same referenced key value, InnoDB performs a foreign key check as if the other parent rows with the same key value do not exist. For example, if you define a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB does not permit the deletion of any of the parent rows.

  • If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the same cascade, it acts like RESTRICT. This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the other hand, is possible, as is a self-referential ON DELETE CASCADE. Cascading operations may not be nested more than 15 levels deep.

  • In an SQL statement that inserts, deletes, or updates many rows, foreign key constraints (like unique constraints) are checked row-by-row. When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records that it must examine. MySQL checks foreign key constraints immediately; the check is not deferred to transaction commit. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the entire SQL statement has been processed. This means that it is not possible to delete a row that refers to itself using a foreign key.

  • No storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-integrity constraint definitions. Use of an explicit MATCH clause does not have the specified effect, and it causes ON DELETE and ON UPDATE clauses to be ignored. Specifying the MATCH should be avoided.

    The MATCH clause in the SQL standard controls how NULL values in a composite (multiple-column) foreign key are handled when comparing to a primary key in the referenced table. MySQL essentially implements the semantics defined by MATCH SIMPLE, which permits a foreign key to be all or partially NULL. In that case, a (child table) row containing such a foreign key can be inserted even though it does not match any row in the referenced (parent) table. (It is possible to implement other semantics using triggers.)

  • MySQL requires that the referenced columns be indexed for performance reasons. However, MySQL does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL.

    A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL but rather an InnoDB extension. The NDB storage engine, on the other hand, requires an explicit unique key (or primary key) on any column referenced as a foreign key.

    The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only UNIQUE (including PRIMARY) and NOT NULL keys.

  • MySQL parses but ignores inline REFERENCES specifications (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification. For storage engines that do not support foreign keys (such as MyISAM), MySQL Server parses and ignores foreign key specifications.

For information about foreign key constraints, see Section, “FOREIGN KEY Constraints”. '--' as the Start of a Comment

Standard SQL uses the C syntax /* this is a comment */ for comments, and MySQL Server supports this syntax as well. MySQL also support extensions to this syntax that enable MySQL-specific SQL to be embedded in the comment, as described in Section 9.7, “Comments”.

Standard SQL uses -- as a start-comment sequence. MySQL Server uses # as the start comment character. MySQL Server also supports a variant of the -- comment style. That is, the -- start-comment sequence must be followed by a space (or by a control character such as a newline). The space is required to prevent problems with automatically generated SQL queries that use constructs such as the following, where we automatically insert the value of the payment for payment:

UPDATE account SET credit=credit-payment

Consider about what happens if payment has a negative value such as -1:

UPDATE account SET credit=credit--1

credit--1 is a valid expression in SQL, but -- is interpreted as the start of a comment, part of the expression is discarded. The result is a statement that has a completely different meaning than intended:

UPDATE account SET credit=credit

The statement produces no change in value at all. This illustrates that permitting comments to start with -- can have serious consequences.

Using our implementation requires a space following the -- for it to be recognized as a start-comment sequence in MySQL Server. Therefore, credit--1 is safe to use.

Another safe feature is that the mysql command-line client ignores lines that start with --.

1.7.3 How MySQL Deals with Constraints

MySQL enables you to work both with transactional tables that permit rollback and with nontransactional tables that do not. Because of this, constraint handling is a bit different in MySQL than in other DBMSs. We must handle the case when you have inserted or updated a lot of rows in a nontransactional table for which changes cannot be rolled back when an error occurs.

The basic philosophy is that MySQL Server tries to produce an error for anything that it can detect while parsing a statement to be executed, and tries to recover from any errors that occur while executing the statement. We do this in most cases, but not yet for all.

The options MySQL has when an error occurs are to stop the statement in the middle or to recover as well as possible from the problem and continue. By default, the server follows the latter course. This means, for example, that the server may coerce invalid values to the closest valid values.

Several SQL mode options are available to provide greater control over handling of bad data values and whether to continue statement execution or abort when errors occur. Using these options, you can configure MySQL Server to act in a more traditional fashion that is like other DBMSs that reject improper input. The SQL mode can be set globally at server startup to affect all clients. Individual clients can set the SQL mode at runtime, which enables each client to select the behavior most appropriate for its requirements. See Section 5.1.11, “Server SQL Modes”.

The following sections describe how MySQL Server handles different types of constraints. PRIMARY KEY and UNIQUE Index Constraints

Normally, errors occur for data-change statements (such as INSERT or UPDATE) that would violate primary-key, unique-key, or foreign-key constraints. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. If you are using a nontransactional storage engine, MySQL stops processing the statement at the row for which the error occurred and leaves any remaining rows unprocessed.

MySQL supports an IGNORE keyword for INSERT, UPDATE, and so forth. If you use it, MySQL ignores primary-key or unique-key violations and continues processing with the next row. See the section for the statement that you are using (Section 13.2.6, “INSERT Statement”, Section 13.2.13, “UPDATE Statement”, and so forth).

You can get information about the number of rows actually inserted or updated with the mysql_info() C API function. You can also use the SHOW WARNINGS statement. See mysql_info(), and Section, “SHOW WARNINGS Statement”.

InnoDB and NDB tables support foreign keys. See Section, “FOREIGN KEY Constraints”. FOREIGN KEY Constraints

Foreign keys let you cross-reference related data across tables, and foreign key constraints help keep this spread-out data consistent.

MySQL supports ON UPDATE and ON DELETE foreign key references in CREATE TABLE and ALTER TABLE statements. The available referential actions are RESTRICT, CASCADE, SET NULL, and NO ACTION (the default).

SET DEFAULT is also supported by the MySQL Server but is currently rejected as invalid by InnoDB. Since MySQL does not support deferred constraint checking, NO ACTION is treated as RESTRICT. For the exact syntax supported by MySQL for foreign keys, see Section, “FOREIGN KEY Constraints”.

MATCH FULL, MATCH PARTIAL, and MATCH SIMPLE are allowed, but their use should be avoided, as they cause the MySQL Server to ignore any ON DELETE or ON UPDATE clause used in the same statement. MATCH options do not have any other effect in MySQL, which in effect enforces MATCH SIMPLE semantics full-time.

MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created.

You can obtain information about foreign keys from the INFORMATION_SCHEMA.KEY_COLUMN_USAGE table. An example of a query against this table is shown here:

| fk1          | myuser        | myuser_id   | f               |
| fk1          | product_order | customer_id | f2              |
| fk1          | product_order | product_id  | f1              |
3 rows in set (0.01 sec)

Information about foreign keys on InnoDB tables can also be found in the INNODB_FOREIGN and INNODB_FOREIGN_COLS tables, in the INFORMATION_SCHEMA database.

InnoDB and NDB tables support foreign keys. Enforced Constraints on Invalid Data

By default, MySQL 8.0 rejects invalid or improper data values and aborts the statement in which they occur. It is possible to alter this behavior to be more forgiving of invalid values, such that the server coerces them to valid ones for data entry, by disabling strict SQL mode (see Section 5.1.11, “Server SQL Modes”), but this is not recommended.

Older versions of MySQL employed the forgiving behavior by default; for a description of this behavior, see Constraints on Invalid Data. ENUM and SET Constraints

ENUM and SET columns provide an efficient way to define columns that can contain only a given set of values. See Section 11.3.5, “The ENUM Type”, and Section 11.3.6, “The SET Type”.

Unless strict mode is disabled (not recommended, but see Section 5.1.11, “Server SQL Modes”), the definition of a ENUM or SET column acts as a constraint on values entered into the column. An error occurs for values that do not satisfy these conditions:

  • An ENUM value must be one of those listed in the column definition, or the internal numeric equivalent thereof. The value cannot be the error value (that is, 0 or the empty string). For a column defined as ENUM('a','b','c'), values such as '', 'd', or 'ax' are invalid and are rejected.

  • A SET value must be the empty string or a value consisting only of the values listed in the column definition separated by commas. For a column defined as SET('a','b','c'), values such as 'd' or 'a,b,c,d' are invalid and are rejected.

Errors for invalid values can be suppressed in strict mode if you use INSERT IGNORE or UPDATE IGNORE. In this case, a warning is generated rather than an error. For ENUM, the value is inserted as the error member (0). For SET, the value is inserted as given except that any invalid substrings are deleted. For example, 'a,x,b,y' results in a value of 'a,b'.

1.8 Credits

The following sections list developers, contributors, and supporters that have helped to make MySQL what it is today.

1.8.1 Contributors to MySQL

Although Oracle Corporation and/or its affiliates own all copyrights in the MySQL server and the MySQL manual, we wish to recognize those who have made contributions of one kind or another to the MySQL distribution. Contributors are listed here, in somewhat random order:

  • Gianmassimo Vigazzola or

    The initial port to Win32/NT.

  • Per Eric Olsson

    For constructive criticism and real testing of the dynamic record format.

  • Irena Pancirov

    Win32 port with Borland compiler. mysqlshutdown.exe and mysqlwatch.exe.

  • David J. Hughes

    For the effort to make a shareware SQL database. At TcX, the predecessor of MySQL AB, we started with mSQL, but found that it couldn't satisfy our purposes so instead we wrote an SQL interface to our application builder Unireg. mysqladmin and mysql client are programs that were largely influenced by their mSQL counterparts. We have put a lot of effort into making the MySQL syntax a superset of mSQL. Many of the API's ideas are borrowed from mSQL to make it easy to port free mSQL programs to the MySQL API. The MySQL software doesn't contain any code from mSQL. Two files in the distribution (client/insert_test.c and client/select_test.c) are based on the corresponding (noncopyrighted) files in the mSQL distribution, but are modified as examples showing the changes necessary to convert code from mSQL to MySQL Server. (mSQL is copyrighted David J. Hughes.)

  • Patrick Lynch

    For helping us acquire

  • Fred Lindberg

    For setting up qmail to handle the MySQL mailing list and for the incredible help we got in managing the MySQL mailing lists.

  • Igor Romanenko

    mysqldump (previously msqldump, but ported and enhanced by Monty).

  • Yuri Dario

    For keeping up and extending the MySQL OS/2 port.

  • Tim Bunce

    Author of mysqlhotcopy.

  • Zarko Mocnik

    Sorting for Slovenian language.

  • "TAMITO"

    The _MB character set macros and the ujis and sjis character sets.

  • Joshua Chamas

    Base for concurrent insert, extended date syntax, debugging on NT, and answering on the MySQL mailing list.

  • Yves Carlier

    mysqlaccess, a program to show the access rights for a user.

  • Rhys Jones (And GWE Technologies Limited)

    For one of the early JDBC drivers.

  • Dr Xiaokun Kelvin ZHU

    Further development of one of the early JDBC drivers and other MySQL-related Java tools.

  • James Cooper

    For setting up a searchable mailing list archive at his site.

  • Rick Mehalick

    For xmysql, a graphical X client for MySQL Server.

  • Doug Sisk

    For providing RPM packages of MySQL for Red Hat Linux.

  • Diemand Alexander V.

    For providing RPM packages of MySQL for Red Hat Linux-Alpha.

  • Antoni Pamies Olive

    For providing RPM versions of a lot of MySQL clients for Intel and SPARC.

  • Jay Bloodworth

    For providing RPM versions for MySQL 3.21.

  • David Sacerdote

    Ideas for secure checking of DNS host names.

  • Wei-Jou Chen

    Some support for Chinese(BIG5) characters.

  • Wei He

    A lot of functionality for the Chinese(GBK) character set.

  • Jan Pazdziora

    Czech sorting order.

  • Zeev Suraski

    FROM_UNIXTIME() time formatting, ENCRYPT() functions, and bison advisor. Active mailing list member.

  • Luuk de Boer

    Ported (and extended) the benchmark suite to DBI/DBD. Have been of great help with crash-me and running benchmarks. Some new date functions. The mysql_setpermission script.

  • Alexis Mikhailov

    User-defined functions (UDFs); CREATE FUNCTION and DROP FUNCTION.

  • Andreas F. Bobak

    The AGGREGATE extension to user-defined functions.

  • Ross Wakelin

    Help to set up InstallShield for MySQL-Win32.

  • Jethro Wright III

    The libmysql.dll library.

  • James Pereria

    Mysqlmanager, a Win32 GUI tool for administering MySQL Servers.

  • Curt Sampson

    Porting of MIT-pthreads to NetBSD/Alpha and NetBSD 1.3/i386.

  • Martin Ramsch

    Examples in the MySQL Tutorial.

  • Steve Harvey

    For making mysqlaccess more secure.

  • Konark IA-64 Centre of Persistent Systems Private Limited

    Help with the Win64 port of the MySQL server.

  • Albert Chin-A-Young.

    Configure updates for Tru64, large file support and better TCP wrappers support.

  • John Birrell

    Emulation of pthread_mutex() for OS/2.

  • Benjamin Pflugmann

    Extended MERGE tables to handle INSERTS. Active member on the MySQL mailing lists.

  • Jocelyn Fournier

    Excellent spotting and reporting innumerable bugs (especially in the MySQL 4.1 subquery code).

  • Marc Liyanage

    Maintaining the OS X packages and providing invaluable feedback on how to create OS X packages.

  • Robert Rutherford

    Providing invaluable information and feedback about the QNX port.

  • Previous developers of NDB Cluster

    Lots of people were involved in various ways summer students, master thesis students, employees. In total more than 100 people so too many to mention here. Notable name is Ataullah Dabaghi who up until 1999 contributed around a third of the code base. A special thanks also to developers of the AXE system which provided much of the architectural foundations for NDB Cluster with blocks, signals and crash tracing functionality. Also credit should be given to those who believed in the ideas enough to allocate of their budgets for its development from 1992 to present time.

  • Google Inc.

    We wish to recognize Google Inc. for contributions to the MySQL distribution: Mark Callaghan's SMP Performance patches and other patches.

Other contributors, bugfinders, and testers: James H. Thompson, Maurizio Menghini, Wojciech Tryc, Luca Berra, Zarko Mocnik, Wim Bonis, Elmar Haneke, , , , Ted Deppner , Mike Simons, Jaakko Hyvatti.

And lots of bug report/patches from the folks on the mailing list.

A big tribute goes to those that help us answer questions on the MySQL mailing lists:

1.8.2 Documenters and translators

The following people have helped us with writing the MySQL documentation and translating the documentation or error messages in MySQL.

  • Kim Aldale

    Helped to rewrite Monty's and David's early attempts at English into English.

  • Michael J. Miller Jr.

    For the first MySQL manual. And a lot of spelling/language fixes for the FAQ (that turned into the MySQL manual a long time ago).

  • Yan Cailin

    First translator of the MySQL Reference Manual into simplified Chinese in early 2000 on which the Big5 and HK coded versions were based.

  • Jay Flaherty

    Big parts of the Perl DBI/DBD section in the manual.

  • Paul Southworth , Ray Loyzaga

    Proof-reading of the Reference Manual.

  • Therrien Gilbert , Jean-Marc Pouyot

    French error messages.

  • Petr Snajdr,

    Czech error messages.

  • Jaroslaw Lewandowski

    Polish error messages.

  • Miguel Angel Fernandez Roiz

    Spanish error messages.

  • Roy-Magne Mo

    Norwegian error messages and testing of MySQL 3.21.xx.

  • Timur I. Bakeyev

    Russian error messages.

  • & Filippo Grassilli

    Italian error messages.

  • Dirk Munzinger

    German error messages.

  • Billik Stefan

    Slovak error messages.

  • Stefan Saroiu

    Romanian error messages.

  • Peter Feher

    Hungarian error messages.

  • Roberto M. Serqueira

    Portuguese error messages.

  • Carsten H. Pedersen

    Danish error messages.

  • Arjen Lentz

    Dutch error messages, completing earlier partial translation (also work on consistency and spelling).

1.8.3 Packages that support MySQL

The following is a list of creators/maintainers of some of the most important API/packages/applications that a lot of people use with MySQL.

We cannot list every possible package here because the list would then be way to hard to maintain. For other packages, please refer to the software portal at

  • Tim Bunce, Alligator Descartes

    For the DBD (Perl) interface.

  • Andreas Koenig

    For the Perl interface for MySQL Server.

  • Jochen Wiedmann

    For maintaining the Perl DBD::mysql module.

  • Eugene Chan

    For porting PHP for MySQL Server.

  • Georg Richter

    MySQL 4.1 testing and bug hunting. New PHP 5.0 mysqli extension (API) for use with MySQL 4.1 and up.

  • Giovanni Maruzzelli

    For porting iODBC (Unix ODBC).

  • Xavier Leroy

    The author of LinuxThreads (used by the MySQL Server on Linux).

1.8.4 Tools that were used to create MySQL

The following is a list of some of the tools we have used to create MySQL. We use this to express our thanks to those that has created them as without these we could not have made MySQL what it is today.

  • Free Software Foundation

    From whom we got an excellent compiler (gcc), an excellent debugger (gdb and the libc library (from which we have borrowed strto.c to get some code working in Linux).

  • Free Software Foundation & The XEmacs development team

    For a really great editor/environment.

  • Julian Seward

    Author of valgrind, an excellent memory checker tool that has helped us find a lot of otherwise hard to find bugs in MySQL.

  • Dorothea Lütkehaus and Andreas Zeller

    For DDD (The Data Display Debugger) which is an excellent graphical front end to gdb).

1.8.5 Supporters of MySQL

Although Oracle Corporation and/or its affiliates own all copyrights in the MySQL server and the MySQL manual, we wish to recognize the following companies, which helped us finance the development of the MySQL server, such as by paying us for developing a new feature or giving us hardware for development of the MySQL server.

  • VA Linux /

    Funded replication.

  • NuSphere

    Editing of the MySQL manual.

  • Stork Design studio

    The MySQL website in use between 1998-2000.

  • Intel

    Contributed to development on Windows and Linux platforms.

  • Compaq

    Contributed to Development on Linux/Alpha.

  • SWSoft

    Development on the embedded mysqld version.

  • FutureQuest

    The --skip-show-database option.