Chapter 22 Stored Programs and Views

This chapter discusses stored programs and views, which are database objects defined in terms of SQL code that is stored on the server for later execution.

Stored programs include these objects:

  • Stored routines, that is, stored procedures and functions. A stored procedure is invoked using the CALL statement. A procedure does not have a return value but can modify its parameters for later inspection by the caller. It can also generate result sets to be returned to the client program. A stored function is used much like a built-in function. you invoke it in an expression and it returns a value during expression evaluation.

  • Triggers. A trigger is a named database object that is associated with a table and that is activated when a particular event occurs for the table, such as an insert or update.

  • Events. An event is a task that the server runs according to schedule.

Views are stored queries that when referenced produce a result set. A view acts as a virtual table.

This chapter describes how to use stored programs and views. The following sections provide additional information about SQL syntax for statements related to these objects:

In MySQL, metadata changes to objects referred to by stored programs are detected and cause automatic reparsing of the affected statements when the program is next executed. For more information, see Section 9.10.4, “Caching of Prepared Statements and Stored Programs”.

22.1 Defining Stored Programs

Each stored program contains a body that consists of an SQL statement. This statement may be a compound statement made up of several statements separated by semicolon (;) characters. For example, the following stored procedure has a body made up of a BEGIN ... END block that contains a SET statement and a REPEAT loop that itself contains another SET statement:

CREATE PROCEDURE dorepeat(p1 INT)
BEGIN
  SET @x = 0;
  REPEAT SET @x = @x + 1; UNTIL @x > p1 END REPEAT;
END;

If you use the mysql client program to define a stored program containing semicolon characters, a problem arises. By default, mysql itself recognizes the semicolon as a statement delimiter, so you must redefine the delimiter temporarily to cause mysql to pass the entire stored program definition to the server.

To redefine the mysql delimiter, use the delimiter command. The following example shows how to do this for the dorepeat() procedure just shown. The delimiter is changed to // to enable the entire definition to be passed to the server as a single statement, and then restored to ; before invoking the procedure. This enables the ; delimiter used in the procedure body to be passed through to the server rather than being interpreted by mysql itself.

mysql> delimiter //

mysql> CREATE PROCEDURE dorepeat(p1 INT)
    -> BEGIN
    ->   SET @x = 0;
    ->   REPEAT SET @x = @x + 1; UNTIL @x > p1 END REPEAT;
    -> END
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;

mysql> CALL dorepeat(1000);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @x;
+------+
| @x   |
+------+
| 1001 |
+------+
1 row in set (0.00 sec)

You can redefine the delimiter to a string other than //, and the delimiter can consist of a single character or multiple characters. You should avoid the use of the backslash (\) character because that is the escape character for MySQL.

The following is an example of a function that takes a parameter, performs an operation using an SQL function, and returns the result. In this case, it is unnecessary to use delimiter because the function definition contains no internal ; statement delimiters:

mysql> CREATE FUNCTION hello (s CHAR(20))
mysql> RETURNS CHAR(50) DETERMINISTIC
    -> RETURN CONCAT('Hello, ',s,'!');
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT hello('world');
+----------------+
| hello('world') |
+----------------+
| Hello, world!  |
+----------------+
1 row in set (0.00 sec)

22.2 Using Stored Routines (Procedures and Functions)

MySQL supports stored routines (procedures and functions). A stored routine is a set of SQL statements that can be stored in the server. Once this has been done, clients don't need to keep reissuing the individual statements but can refer to the stored routine instead.

Stored routines require the proc table in the mysql database. This table is created during the MySQL installation procedure. If you are upgrading to MySQL 5.7 from an earlier version, be sure to update your grant tables to make sure that the proc table exists. See Section 5.4.7, “mysql_upgrade — Check and Upgrade MySQL Tables”.

Stored routines can be particularly useful in certain situations:

  • When multiple client applications are written in different languages or work on different platforms, but need to perform the same database operations.

  • When security is paramount. Banks, for example, use stored procedures and functions for all common operations. This provides a consistent and secure environment, and routines can ensure that each operation is properly logged. In such a setup, applications and users would have no access to the database tables directly, but can only execute specific stored routines.

Stored routines can provide improved performance because less information needs to be sent between the server and the client. The tradeoff is that this does increase the load on the database server because more of the work is done on the server side and less is done on the client (application) side. Consider this if many client machines (such as Web servers) are serviced by only one or a few database servers.

Stored routines also enable you to have libraries of functions in the database server. This is a feature shared by modern application languages that enable such design internally (for example, by using classes). Using these client application language features is beneficial for the programmer even outside the scope of database use.

MySQL follows the SQL:2003 syntax for stored routines, which is also used by IBM's DB2. All syntax described here is supported and any limitations and extensions are documented where appropriate.

Additional Resources

22.2.1 Stored Routine Syntax

A stored routine is either a procedure or a function. Stored routines are created with the CREATE PROCEDURE and CREATE FUNCTION statements (see Section 14.1.16, “CREATE PROCEDURE and CREATE FUNCTION Syntax”). A procedure is invoked using a CALL statement (see Section 14.2.1, “CALL Syntax”), and can only pass back values using output variables. A function can be called from inside a statement just like any other function (that is, by invoking the function's name), and can return a scalar value. The body of a stored routine can use compound statements (see Section 14.6, “Compound-Statement Syntax”).

Stored routines can be dropped with the DROP PROCEDURE and DROP FUNCTION statements (see Section 14.1.27, “DROP PROCEDURE and DROP FUNCTION Syntax”), and altered with the ALTER PROCEDURE and ALTER FUNCTION statements (see Section 14.1.6, “ALTER PROCEDURE Syntax”).

A stored procedure or function is associated with a particular database. This has several implications:

  • When the routine is invoked, an implicit USE db_name is performed (and undone when the routine terminates). USE statements within stored routines are not permitted.

  • You can qualify routine names with the database name. This can be used to refer to a routine that is not in the current database. For example, to invoke a stored procedure p or function f that is associated with the test database, you can say CALL test.p() or test.f().

  • When a database is dropped, all stored routines associated with it are dropped as well.

Stored functions cannot be recursive.

Recursion in stored procedures is permitted but disabled by default. To enable recursion, set the max_sp_recursion_depth server system variable to a value greater than zero. Stored procedure recursion increases the demand on thread stack space. If you increase the value of max_sp_recursion_depth, it may be necessary to increase thread stack size by increasing the value of thread_stack at server startup. See Section 6.1.5, “Server System Variables”, for more information.

MySQL supports a very useful extension that enables the use of regular SELECT statements (that is, without using cursors or local variables) inside a stored procedure. The result set of such a query is simply sent directly to the client. Multiple SELECT statements generate multiple result sets, so the client must use a MySQL client library that supports multiple result sets. This means the client must use a client library from a version of MySQL at least as recent as 4.1. The client should also specify the CLIENT_MULTI_RESULTS option when it connects. For C programs, this can be done with the mysql_real_connect() C API function. See Section 26.8.7.54, “mysql_real_connect()”, and Section 26.8.17, “C API Support for Multiple Statement Execution”.

22.2.2 Stored Routines and MySQL Privileges

The MySQL grant system takes stored routines into account as follows:

  • The CREATE ROUTINE privilege is needed to create stored routines.

  • The ALTER ROUTINE privilege is needed to alter or drop stored routines. This privilege is granted automatically to the creator of a routine if necessary, and dropped from the creator when the routine is dropped.

  • The EXECUTE privilege is required to execute stored routines. However, this privilege is granted automatically to the creator of a routine if necessary (and dropped from the creator when the routine is dropped). Also, the default SQL SECURITY characteristic for a routine is DEFINER, which enables users who have access to the database with which the routine is associated to execute the routine.

  • If the automatic_sp_privileges system variable is 0, the EXECUTE and ALTER ROUTINE privileges are not automatically granted to and dropped from the routine creator.

  • The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.

The server manipulates the mysql.proc table in response to statements that create, alter, or drop stored routines. It is not supported that the server will notice manual manipulation of this table.

22.2.3 Stored Routine Metadata

Metadata about stored routines can be obtained as follows:

22.2.4 Stored Procedures, Functions, Triggers, and LAST_INSERT_ID()

Within the body of a stored routine (procedure or function) or a trigger, the value of LAST_INSERT_ID() changes the same way as for statements executed outside the body of these kinds of objects (see Section 13.14, “Information Functions”). The effect of a stored routine or trigger upon the value of LAST_INSERT_ID() that is seen by following statements depends on the kind of routine:

  • If a stored procedure executes statements that change the value of LAST_INSERT_ID(), the changed value is seen by statements that follow the procedure call.

  • For stored functions and triggers that change the value, the value is restored when the function or trigger ends, so following statements do not see a changed value.

22.3 Using Triggers

A trigger is a named database object that is associated with a table, and that activates when a particular event occurs for the table. Some uses for triggers are to perform checks of values to be inserted into a table or to perform calculations on values involved in an update.

A trigger is defined to activate when a statement inserts, updates, or deletes rows in the associated table. These row operations are trigger events. For example, rows can be inserted by INSERT or LOAD DATA statements, and an insert trigger activates for each inserted row. A trigger can be set to activate either before or after the trigger event. For example, you can have a trigger activate before each row that is inserted into a table or after each row that is updated.

Important

MySQL triggers activate only for changes made to tables by SQL statements. They do not activate for changes in views, nor by changes to tables made by APIs that do not transmit SQL statements to the MySQL Server. This means that triggers are not activated in the following two cases:

  • Triggers are not activated by changes in INFORMATION_SCHEMA or performance_schema tables, because these tables are actually views.

  • Triggers are not activated by updates made using the NDB API.

The following sections describe the syntax for creating and dropping triggers, show some examples of how to use them, and indicate how to obtain trigger metadata.

Additional Resources

22.3.1 Trigger Syntax and Examples

To create a trigger or drop a trigger, use the CREATE TRIGGER or DROP TRIGGER statement, described in Section 14.1.20, “CREATE TRIGGER Syntax”, and Section 14.1.31, “DROP TRIGGER Syntax”.

Here is a simple example that associates a trigger with a table, to activate for INSERT operations. The trigger acts as an accumulator, summing the values inserted into one of the columns of the table.

mysql> CREATE TABLE account (acct_num INT, amount DECIMAL(10,2));
Query OK, 0 rows affected (0.03 sec)

mysql> CREATE TRIGGER ins_sum BEFORE INSERT ON account
    -> FOR EACH ROW SET @sum = @sum + NEW.amount;
Query OK, 0 rows affected (0.06 sec)

The CREATE TRIGGER statement creates a trigger named ins_sum that is associated with the account table. It also includes clauses that specify the trigger action time, the triggering event, and what to do when the trigger activates:

  • The keyword BEFORE indicates the trigger action time. In this case, the trigger activates before each row inserted into the table. The other permitted keyword here is AFTER.

  • The keyword INSERT indicates the trigger event; that is, the type of operation that activates the trigger. In the example, INSERT operations cause trigger activation. You can also create triggers for DELETE and UPDATE operations.

  • The statement following FOR EACH ROW defines the trigger body; that is, the statement to execute each time the trigger activates, which occurs once for each row affected by the triggering event. In the example, the trigger body is a simple SET that accumulates into a user variable the values inserted into the amount column. The statement refers to the column as NEW.amount which means the value of the amount column to be inserted into the new row.

To use the trigger, set the accumulator variable to zero, execute an INSERT statement, and then see what value the variable has afterward:

mysql> SET @sum = 0;
mysql> INSERT INTO account VALUES(137,14.98),(141,1937.50),(97,-100.00);
mysql> SELECT @sum AS 'Total amount inserted';
+-----------------------+
| Total amount inserted |
+-----------------------+
| 1852.48               |
+-----------------------+

In this case, the value of @sum after the INSERT statement has executed is 14.98 + 1937.50 - 100, or 1852.48.

To destroy the trigger, use a DROP TRIGGER statement. You must specify the schema name if the trigger is not in the default schema:

mysql> DROP TRIGGER test.ins_sum;

If you drop a table, any triggers for the table are also dropped.

Trigger names exist in the schema namespace, meaning that all triggers must have unique names within a schema. Triggers in different schemas can have the same name.

As of MySQL 5.7.2, it is possible to define multiple triggers for a given table that have the same trigger event and action time. For example, you can have two BEFORE UPDATE triggers for a table. By default, triggers that have the same trigger event and action time activate in the order they were created. To affect trigger order, specify a clause after FOR EACH ROW that indicates FOLLOWS or PRECEDES and the name of an existing trigger that also has the same trigger event and action time. With FOLLOWS, the new trigger activates after the existing trigger. With PRECEDES, the new trigger activates before the existing trigger.

For example, the following trigger definition defines another BEFORE INSERT trigger for the account table:

mysql> CREATE TRIGGER ins_transaction BEFORE INSERT ON account
    -> FOR EACH ROW PRECEDES ins_sum
    -> SET
    -> @deposits = @deposits + IF(NEW.amount>0,NEW.amount,0),
    -> @withdrawals = @withdrawals + IF(NEW.amount<0,-NEW.amount,0);
Query OK, 0 rows affected (0.02 sec)

This trigger, ins_transaction, is similar to ins_sum but accumulates deposits and withdrawals separately. It has a PRECEDES clause that causes it to activate before ins_sum; without that clause, it would activate after ins_sum because it is created after ins_sum.

Before MySQL 5.7.2, there cannot be multiple triggers for a given table that have the same trigger event and action time. For example, you cannot have two BEFORE UPDATE triggers for a table. To work around this, you can define a trigger that executes multiple statements by using the BEGIN ... END compound statement construct after FOR EACH ROW. (An example appears later in this section.)

Within the trigger body, the OLD and NEW keywords enable you to access columns in the rows affected by a trigger. OLD and NEW are MySQL extensions to triggers; they are not case sensitive.

In an INSERT trigger, only NEW.col_name can be used; there is no old row. In a DELETE trigger, only OLD.col_name can be used; there is no new row. In an UPDATE trigger, you can use OLD.col_name to refer to the columns of a row before it is updated and NEW.col_name to refer to the columns of the row after it is updated.

A column named with OLD is read only. You can refer to it (if you have the SELECT privilege), but not modify it. You can refer to a column named with NEW if you have the SELECT privilege for it. In a BEFORE trigger, you can also change its value with SET NEW.col_name = value if you have the UPDATE privilege for it. This means you can use a trigger to modify the values to be inserted into a new row or used to update a row. (Such a SET statement has no effect in an AFTER trigger because the row change will have already occurred.)

In a BEFORE trigger, the NEW value for an AUTO_INCREMENT column is 0, not the sequence number that is generated automatically when the new row actually is inserted.

By using the BEGIN ... END construct, you can define a trigger that executes multiple statements. Within the BEGIN block, you also can use other syntax that is permitted within stored routines such as conditionals and loops. However, just as for stored routines, if you use the mysql program to define a trigger that executes multiple statements, it is necessary to redefine the mysql statement delimiter so that you can use the ; statement delimiter within the trigger definition. The following example illustrates these points. It defines an UPDATE trigger that checks the new value to be used for updating each row, and modifies the value to be within the range from 0 to 100. This must be a BEFORE trigger because the value must be checked before it is used to update the row:

mysql> delimiter //
mysql> CREATE TRIGGER upd_check BEFORE UPDATE ON account
    -> FOR EACH ROW
    -> BEGIN
    ->     IF NEW.amount < 0 THEN
    ->         SET NEW.amount = 0;
    ->     ELSEIF NEW.amount > 100 THEN
    ->         SET NEW.amount = 100;
    ->     END IF;
    -> END;//
mysql> delimiter ;

It can be easier to define a stored procedure separately and then invoke it from the trigger using a simple CALL statement. This is also advantageous if you want to execute the same code from within several triggers.

There are limitations on what can appear in statements that a trigger executes when activated:

  • The trigger cannot use the CALL statement to invoke stored procedures that return data to the client or that use dynamic SQL. (Stored procedures are permitted to return data to the trigger through OUT or INOUT parameters.)

  • The trigger cannot use statements that explicitly or implicitly begin or end a transaction, such as START TRANSACTION, COMMIT, or ROLLBACK. (ROLLBACK to SAVEPOINT is permitted because it does not end a transaction.).

See also Section C.1, “Restrictions on Stored Programs”.

MySQL handles errors during trigger execution as follows:

  • If a BEFORE trigger fails, the operation on the corresponding row is not performed.

  • A BEFORE trigger is activated by the attempt to insert or modify the row, regardless of whether the attempt subsequently succeeds.

  • An AFTER trigger is executed only if any BEFORE triggers and the row operation execute successfully.

  • An error during either a BEFORE or AFTER trigger results in failure of the entire statement that caused trigger invocation.

  • For transactional tables, failure of a statement should cause rollback of all changes performed by the statement. Failure of a trigger causes the statement to fail, so trigger failure also causes rollback. For nontransactional tables, such rollback cannot be done, so although the statement fails, any changes performed prior to the point of the error remain in effect.

Triggers can contain direct references to tables by name, such as the trigger named testref shown in this example:

CREATE TABLE test1(a1 INT);
CREATE TABLE test2(a2 INT);
CREATE TABLE test3(a3 INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
CREATE TABLE test4(
  a4 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  b4 INT DEFAULT 0
);

delimiter |

CREATE TRIGGER testref BEFORE INSERT ON test1
  FOR EACH ROW
  BEGIN
    INSERT INTO test2 SET a2 = NEW.a1;
    DELETE FROM test3 WHERE a3 = NEW.a1;
    UPDATE test4 SET b4 = b4 + 1 WHERE a4 = NEW.a1;
  END;
|

delimiter ;

INSERT INTO test3 (a3) VALUES
  (NULL), (NULL), (NULL), (NULL), (NULL),
  (NULL), (NULL), (NULL), (NULL), (NULL);

INSERT INTO test4 (a4) VALUES
  (0), (0), (0), (0), (0), (0), (0), (0), (0), (0);

Suppose that you insert the following values into table test1 as shown here:

mysql> INSERT INTO test1 VALUES 
    -> (1), (3), (1), (7), (1), (8), (4), (4);
Query OK, 8 rows affected (0.01 sec)
Records: 8  Duplicates: 0  Warnings: 0

As a result, the four tables contain the following data:

mysql> SELECT * FROM test1;
+------+
| a1   |
+------+
|    1 |
|    3 |
|    1 |
|    7 |
|    1 |
|    8 |
|    4 |
|    4 |
+------+
8 rows in set (0.00 sec)

mysql> SELECT * FROM test2;
+------+
| a2   |
+------+
|    1 |
|    3 |
|    1 |
|    7 |
|    1 |
|    8 |
|    4 |
|    4 |
+------+
8 rows in set (0.00 sec)

mysql> SELECT * FROM test3;
+----+
| a3 |
+----+
|  2 |
|  5 |
|  6 |
|  9 |
| 10 |
+----+
5 rows in set (0.00 sec)

mysql> SELECT * FROM test4;
+----+------+
| a4 | b4   |
+----+------+
|  1 |    3 |
|  2 |    0 |
|  3 |    1 |
|  4 |    2 |
|  5 |    0 |
|  6 |    0 |
|  7 |    1 |
|  8 |    1 |
|  9 |    0 |
| 10 |    0 |
+----+------+
10 rows in set (0.00 sec)

22.3.2 Trigger Metadata

Metadata about triggers can be obtained as follows:

22.4 Using the Event Scheduler

The MySQL Event Scheduler manages the scheduling and execution of events, that is, tasks that run according to a schedule. The following discussion covers the Event Scheduler and is divided into the following sections:

Stored routines require the event table in the mysql database. This table is created during the MySQL 5.7 installation procedure. If you are upgrading to MySQL 5.7 from an earlier version, be sure to update your grant tables to make sure that the event table exists. See Section 5.4.7, “mysql_upgrade — Check and Upgrade MySQL Tables”.

Additional Resources

22.4.1 Event Scheduler Overview

MySQL Events are tasks that run according to a schedule. Therefore, we sometimes refer to them as scheduled events. When you create an event, you are creating a named database object containing one or more SQL statements to be executed at one or more regular intervals, beginning and ending at a specific date and time. Conceptually, this is similar to the idea of the Unix crontab (also known as a cron job) or the Windows Task Scheduler.

Scheduled tasks of this type are also sometimes known as temporal triggers, implying that these are objects that are triggered by the passage of time. While this is essentially correct, we prefer to use the term events to avoid confusion with triggers of the type discussed in Section 22.3, “Using Triggers”. Events should more specifically not be confused with temporary triggers. Whereas a trigger is a database object whose statements are executed in response to a specific type of event that occurs on a given table, a (scheduled) event is an object whose statements are executed in response to the passage of a specified time interval.

While there is no provision in the SQL Standard for event scheduling, there are precedents in other database systems, and you may notice some similarities between these implementations and that found in the MySQL Server.

MySQL Events have the following major features and properties:

  • In MySQL, an event is uniquely identified by its name and the schema to which it is assigned.

  • An event performs a specific action according to a schedule. This action consists of an SQL statement, which can be a compound statement in a BEGIN ... END block if desired (see Section 14.6, “Compound-Statement Syntax”). An event's timing can be either one-time or recurrent. A one-time event executes one time only. A recurrent event repeats its action at a regular interval, and the schedule for a recurring event can be assigned a specific start day and time, end day and time, both, or neither. (By default, a recurring event's schedule begins as soon as it is created, and continues indefinitely, until it is disabled or dropped.)

    If a repeating event does not terminate within its scheduling interval, the result may be multiple instances of the event executing simultaneously. If this is undesirable, you should institute a mechanism to prevent simultaneous instances. For example, you could use the GET_LOCK() function, or row or table locking.

  • Users can create, modify, and drop scheduled events using SQL statements intended for these purposes. Syntactically invalid event creation and modification statements fail with an appropriate error message. A user may include statements in an event's action which require privileges that the user does not actually have. The event creation or modification statement succeeds but the event's action fails. See Section 22.4.6, “The Event Scheduler and MySQL Privileges” for details.

  • Many of the properties of an event can be set or modified using SQL statements. These properties include the event's name, timing, persistence (that is, whether it is preserved following the expiration of its schedule), status (enabled or disabled), action to be performed, and the schema to which it is assigned. See Section 14.1.2, “ALTER EVENT Syntax”.

    The default definer of an event is the user who created the event, unless the event has been altered, in which case the definer is the user who issued the last ALTER EVENT statement affecting that event. An event can be modified by any user having the EVENT privilege on the database for which the event is defined. See Section 22.4.6, “The Event Scheduler and MySQL Privileges”.

  • An event's action statement may include most SQL statements permitted within stored routines. For restrictions, see Section C.1, “Restrictions on Stored Programs”.

22.4.2 Event Scheduler Configuration

Events are executed by a special event scheduler thread; when we refer to the Event Scheduler, we actually refer to this thread. When running, the event scheduler thread and its current state can be seen by users having the PROCESS privilege in the output of SHOW PROCESSLIST, as shown in the discussion that follows.

The global event_scheduler system variable determines whether the Event Scheduler is enabled and running on the server. It has one of these 3 values, which affect event scheduling as described here:

  • OFF: The Event Scheduler is stopped. The event scheduler thread does not run, is not shown in the output of SHOW PROCESSLIST, and no scheduled events are executed. OFF is the default value for event_scheduler.

    When the Event Scheduler is stopped (event_scheduler is OFF), it can be started by setting the value of event_scheduler to ON. (See next item.)

  • ON: The Event Scheduler is started; the event scheduler thread runs and executes all scheduled events.

    When the Event Scheduler is ON, the event scheduler thread is listed in the output of SHOW PROCESSLIST as a daemon process, and its state is represented as shown here:

    mysql> SHOW PROCESSLIST\G
    *************************** 1. row ***************************
         Id: 1
       User: root
       Host: localhost
         db: NULL
    Command: Query
       Time: 0
      State: NULL
       Info: show processlist
    *************************** 2. row ***************************
         Id: 2
       User: event_scheduler
       Host: localhost
         db: NULL
    Command: Daemon
       Time: 3
      State: Waiting for next activation
       Info: NULL
    2 rows in set (0.00 sec)
    

    Event scheduling can be stopped by setting the value of event_scheduler to OFF.

  • DISABLED: This value renders the Event Scheduler nonoperational. When the Event Scheduler is DISABLED, the event scheduler thread does not run (and so does not appear in the output of SHOW PROCESSLIST). In addition, the Event Scheduler state cannot be changed at runtime.

If the Event Scheduler status has not been set to DISABLED, event_scheduler can be toggled between ON and OFF (using SET). It is also possible to use 0 for OFF, and 1 for ON when setting this variable. Thus, any of the following 4 statements can be used in the mysql client to turn on the Event Scheduler:

SET GLOBAL event_scheduler = ON;
SET @@global.event_scheduler = ON;
SET GLOBAL event_scheduler = 1;
SET @@global.event_scheduler = 1;

Similarly, any of these 4 statements can be used to turn off the Event Scheduler:

SET GLOBAL event_scheduler = OFF;
SET @@global.event_scheduler = OFF;
SET GLOBAL event_scheduler = 0;
SET @@global.event_scheduler = 0;

Although ON and OFF have numeric equivalents, the value displayed for event_scheduler by SELECT or SHOW VARIABLES is always one of OFF, ON, or DISABLED. DISABLED has no numeric equivalent. For this reason, ON and OFF are usually preferred over 1 and 0 when setting this variable.

Note that attempting to set event_scheduler without specifying it as a global variable causes an error:

mysql< SET @@event_scheduler = OFF;
ERROR 1229 (HY000): Variable 'event_scheduler' is a GLOBAL
variable and should be set with SET GLOBAL
Important

It is possible to set the Event Scheduler to DISABLED only at server startup. If event_scheduler is ON or OFF, you cannot set it to DISABLED at runtime. Also, if the Event Scheduler is set to DISABLED at startup, you cannot change the value of event_scheduler at runtime.

To disable the event scheduler, use one of the following two methods:

  • As a command-line option when starting the server:

    --event-scheduler=DISABLED
    
  • In the server configuration file (my.cnf, or my.ini on Windows systems), include the line where it will be read by the server (for example, in a [mysqld] section):

    event_scheduler=DISABLED
    

To enable the Event Scheduler, restart the server without the --event-scheduler=DISABLED command-line option, or after removing or commenting out the line containing event-scheduler=DISABLED in the server configuration file, as appropriate. Alternatively, you can use ON (or 1) or OFF (or 0) in place of the DISABLED value when starting the server.

Note

You can issue event-manipulation statements when event_scheduler is set to DISABLED. No warnings or errors are generated in such cases (provided that the statements are themselves valid). However, scheduled events cannot execute until this variable is set to ON (or 1). Once this has been done, the event scheduler thread executes all events whose scheduling conditions are satisfied.

Starting the MySQL server with the --skip-grant-tables option causes event_scheduler to be set to DISABLED, overriding any other value set either on the command line or in the my.cnf or my.ini file (Bug #26807).

For SQL statements used to create, alter, and drop events, see Section 22.4.3, “Event Syntax”.

MySQL provides an EVENTS table in the INFORMATION_SCHEMA database. This table can be queried to obtain information about scheduled events which have been defined on the server. See Section 22.4.4, “Event Metadata”, and Section 23.7, “The INFORMATION_SCHEMA EVENTS Table”, for more information.

For information regarding event scheduling and the MySQL privilege system, see Section 22.4.6, “The Event Scheduler and MySQL Privileges”.

22.4.3 Event Syntax

MySQL provides several SQL statements for working with scheduled events:

22.4.4 Event Metadata

Metadata about events can be obtained as follows:

Event Scheduler Time Representation

Each session in MySQL has a session time zone (STZ). This is the session time_zone value that is initialized from the server's global time_zone value when the session begins but may be changed during the session.

The session time zone that is current when a CREATE EVENT or ALTER EVENT statement executes is used to interpret times specified in the event definition. This becomes the event time zone (ETZ); that is, the time zone that is used for event scheduling and is in effect within the event as it executes.

For representation of event information in the mysql.event table, the execute_at, starts, and ends times are converted to UTC and stored along with the event time zone. This enables event execution to proceed as defined regardless of any subsequent changes to the server time zone or daylight saving time effects. The last_executed time is also stored in UTC.

If you select information from mysql.event, the times just mentioned are retrieved as UTC values. These times can also be obtained by selecting from the INFORMATION_SCHEMA.EVENTS table or from SHOW EVENTS, but they are reported as ETZ values. Other times available from these sources indicate when an event was created or last altered; these are displayed as STZ values. The following table summarizes representation of event times.

Valuemysql.eventINFORMATION_SCHEMA.EVENTSSHOW EVENTS
Execute atUTCETZETZ
StartsUTCETZETZ
EndsUTCETZETZ
Last executedUTCETZn/a
CreatedSTZSTZn/a
Last alteredSTZSTZn/a

22.4.5 Event Scheduler Status

The Event Scheduler writes information about event execution that terminates with an error or warning to the MySQL Server's error log. See Section 22.4.6, “The Event Scheduler and MySQL Privileges” for an example.

To obtain information about the state of the Event Scheduler for debugging and troubleshooting purposes, run mysqladmin debug (see Section 5.5.2, “mysqladmin — Client for Administering a MySQL Server”); after running this command, the server's error log contains output relating to the Event Scheduler, similar to what is shown here:

Events status:
LLA = Last Locked At  LUA = Last Unlocked At
WOC = Waiting On Condition  DL = Data Locked

Event scheduler status:
State      : INITIALIZED
Thread id  : 0
LLA        : init_scheduler:313
LUA        : init_scheduler:318
WOC        : NO
Workers    : 0
Executed   : 0
Data locked: NO

Event queue status:
Element count   : 1
Data locked     : NO
Attempting lock : NO
LLA             : init_queue:148
LUA             : init_queue:168
WOC             : NO
Next activation : 0000-00-00 00:00:00

In statements that occur as part of events executed by the Event Scheduler, diagnostics messages (not only errors, but also warnings) are written to the error log, and, on Windows, to the application event log. For frequently executed events, it is possible for this to result in many logged messages. For example, for SELECT ... INTO var_list statements, if the query returns no rows, a warning with error code 1329 occurs (No data), and the variable values remain unchanged. If the query returns multiple rows, error 1172 occurs (Result consisted of more than one row). For either condition, you can avoid having the warnings be logged by declaring a condition handler; see Section 14.6.7.2, “DECLARE ... HANDLER Syntax”. For statements that may retrieve multiple rows, another strategy is to use LIMIT 1 to limit the result set to a single row.

22.4.6 The Event Scheduler and MySQL Privileges

To enable or disable the execution of scheduled events, it is necessary to set the value of the global event_scheduler system variable. This requires the SUPER privilege.

The EVENT privilege governs the creation, modification, and deletion of events. This privilege can be bestowed using GRANT. For example, this GRANT statement confers the EVENT privilege for the schema named myschema on the user jon@ghidora:

GRANT EVENT ON myschema.* TO jon@ghidora;

(We assume that this user account already exists, and that we wish for it to remain unchanged otherwise.)

To grant this same user the EVENT privilege on all schemas, use the following statement:

GRANT EVENT ON *.* TO jon@ghidora;

The EVENT privilege has global or schema-level scope. Therefore, trying to grant it on a single table results in an error as shown:

mysql> GRANT EVENT ON myschema.mytable TO jon@ghidora;
ERROR 1144 (42000): Illegal GRANT/REVOKE command; please
consult the manual to see which privileges can be used

It is important to understand that an event is executed with the privileges of its definer, and that it cannot perform any actions for which its definer does not have the requisite privileges. For example, suppose that jon@ghidora has the EVENT privilege for myschema. Suppose also that this user has the SELECT privilege for myschema, but no other privileges for this schema. It is possible for jon@ghidora to create a new event such as this one:

CREATE EVENT e_store_ts
    ON SCHEDULE
      EVERY 10 SECOND
    DO
      INSERT INTO myschema.mytable VALUES (UNIX_TIMESTAMP());

The user waits for a minute or so, and then performs a SELECT * FROM mytable; query, expecting to see several new rows in the table. Instead, the table is empty. Since the user does not have the INSERT privilege for the table in question, the event has no effect.

If you inspect the MySQL error log (hostname.err), you can see that the event is executing, but the action it is attempting to perform fails:

2013-09-24T12:41:31.261992Z 25 [ERROR] Event Scheduler:
[jon@ghidora][cookbook.e_store_ts] INSERT command denied to user
'jon'@'ghidora' for table 'mytable'
2013-09-24T12:41:31.262022Z 25 [Note] Event Scheduler:
[jon@ghidora].[myschema.e_store_ts] event execution failed.
2013-09-24T12:41:41.271796Z 26 [ERROR] Event Scheduler:
[jon@ghidora][cookbook.e_store_ts] INSERT command denied to user
'jon'@'ghidora' for table 'mytable'
2013-09-24T12:41:41.272761Z 26 [Note] Event Scheduler:
[jon@ghidora].[myschema.e_store_ts] event execution failed.

Since this user very likely does not have access to the error log, it is possible to verify whether the event's action statement is valid by executing it directly:

mysql> INSERT INTO myschema.mytable VALUES (UNIX_TIMESTAMP());
ERROR 1142 (42000): INSERT command denied to user
'jon'@'ghidora' for table 'mytable'

Inspection of the INFORMATION_SCHEMA.EVENTS table shows that e_store_ts exists and is enabled, but its LAST_EXECUTED column is NULL:

mysql> SELECT * FROM INFORMATION_SCHEMA.EVENTS
     >     WHERE EVENT_NAME='e_store_ts'
     >     AND EVENT_SCHEMA='myschema'\G
*************************** 1. row ***************************
   EVENT_CATALOG: NULL
    EVENT_SCHEMA: myschema
      EVENT_NAME: e_store_ts
         DEFINER: jon@ghidora
      EVENT_BODY: SQL
EVENT_DEFINITION: INSERT INTO myschema.mytable VALUES (UNIX_TIMESTAMP())
      EVENT_TYPE: RECURRING
      EXECUTE_AT: NULL
  INTERVAL_VALUE: 5
  INTERVAL_FIELD: SECOND
        SQL_MODE: NULL
          STARTS: 0000-00-00 00:00:00
            ENDS: 0000-00-00 00:00:00
          STATUS: ENABLED
   ON_COMPLETION: NOT PRESERVE
         CREATED: 2006-02-09 22:36:06
    LAST_ALTERED: 2006-02-09 22:36:06
   LAST_EXECUTED: NULL
   EVENT_COMMENT:
1 row in set (0.00 sec)

To rescind the EVENT privilege, use the REVOKE statement. In this example, the EVENT privilege on the schema myschema is removed from the jon@ghidora user account:

REVOKE EVENT ON myschema.* FROM jon@ghidora;
Important

Revoking the EVENT privilege from a user does not delete or disable any events that may have been created by that user.

An event is not migrated or dropped as a result of renaming or dropping the user who created it.

Suppose that the user jon@ghidora has been granted the EVENT and INSERT privileges on the myschema schema. This user then creates the following event:

CREATE EVENT e_insert
    ON SCHEDULE
      EVERY 7 SECOND
    DO
      INSERT INTO myschema.mytable;

After this event has been created, root revokes the EVENT privilege for jon@ghidora. However, e_insert continues to execute, inserting a new row into mytable each seven seconds. The same would be true if root had issued either of these statements:

  • DROP USER jon@ghidora;

  • RENAME USER jon@ghidora TO someotherguy@ghidora;

You can verify that this is true by examining the mysql.event table (discussed later in this section) or the INFORMATION_SCHEMA.EVENTS table (see Section 23.7, “The INFORMATION_SCHEMA EVENTS Table”) before and after issuing a DROP USER or RENAME USER statement.

Event definitions are stored in the mysql.event table. To drop an event created by another user account, the MySQL root user (or another user with the necessary privileges) can delete rows from this table. For example, to remove the event e_insert shown previously, root can use the following statement:

DELETE FROM mysql.event
    WHERE db = 'myschema'
      AND definer = 'jon@ghidora'
      AND name = 'e_insert';

It is very important to match the event name, database schema name, and user account when deleting rows from the mysql.event table. This is because the same user can create different events of the same name in different schemas.

Users' EVENT privileges are stored in the Event_priv columns of the mysql.user and mysql.db tables. In both cases, this column holds one of the values 'Y' or 'N'. 'N' is the default. mysql.user.Event_priv is set to 'Y' for a given user only if that user has the global EVENT privilege (that is, if the privilege was bestowed using GRANT EVENT ON *.*). For a schema-level EVENT privilege, GRANT creates a row in mysql.db and sets that row's Db column to the name of the schema, the User column to the name of the user, and the Event_priv column to 'Y'. There should never be any need to manipulate these tables directly, since the GRANT EVENT and REVOKE EVENT statements perform the required operations on them.

Five status variables provide counts of event-related operations (but not of statements executed by events; see Section C.1, “Restrictions on Stored Programs”). These are:

  • Com_create_event: The number of CREATE EVENT statements executed since the last server restart.

  • Com_alter_event: The number of ALTER EVENT statements executed since the last server restart.

  • Com_drop_event: The number of DROP EVENT statements executed since the last server restart.

  • Com_show_create_event: The number of SHOW CREATE EVENT statements executed since the last server restart.

  • Com_show_events: The number of SHOW EVENTS statements executed since the last server restart.

You can view current values for all of these at one time by running the statement SHOW STATUS LIKE '%event%';.

22.5 Using Views

MySQL supports views, including updatable views. Views are stored queries that when invoked produce a result set. A view acts as a virtual table.

The following discussion describes the syntax for creating and dropping views, and shows some examples of how to use them.

Additional Resources

22.5.1 View Syntax

The CREATE VIEW statement creates a new view (see Section 14.1.21, “CREATE VIEW Syntax”). To alter the definition of a view or drop a view, use ALTER VIEW (see Section 14.1.10, “ALTER VIEW Syntax”), or DROP VIEW (see Section 14.1.32, “DROP VIEW Syntax”).

A view can be created from many kinds of SELECT statements. It can refer to base tables or other views. It can use joins, UNION, and subqueries. The SELECT need not even refer to any tables. The following example defines a view that selects two columns from another table, as well as an expression calculated from those columns:

mysql> CREATE TABLE t (qty INT, price INT);
mysql> INSERT INTO t VALUES(3, 50), (5, 60);
mysql> CREATE VIEW v AS SELECT qty, price, qty*price AS value FROM t;
mysql> SELECT * FROM v;
+------+-------+-------+
| qty  | price | value |
+------+-------+-------+
|    3 |    50 |   150 |
|    5 |    60 |   300 |
+------+-------+-------+
mysql> SELECT * FROM v WHERE qty = 5;
+------+-------+-------+
| qty  | price | value |
+------+-------+-------+
|    5 |    60 |   300 |
+------+-------+-------+

22.5.2 View Processing Algorithms

The optional ALGORITHM clause for CREATE VIEW or ALTER VIEW is a MySQL extension to standard SQL. It affects how MySQL processes the view. ALGORITHM takes three values: MERGE, TEMPTABLE, or UNDEFINED.

  • For MERGE, the text of a statement that refers to the view and the view definition are merged such that parts of the view definition replace corresponding parts of the statement.

  • For TEMPTABLE, the results from the view are retrieved into a temporary table, which then is used to execute the statement.

  • For UNDEFINED, MySQL chooses which algorithm to use. It prefers MERGE over TEMPTABLE if possible, because MERGE is usually more efficient and because a view cannot be updatable if a temporary table is used.

  • If no ALGORITHM clause is present, UNDEFINED is the default algorithm prior to MySQL 5.7.6. As of 5.7.6, the default algorithm is determined by the value of the derived_merge flag of the optimizer_switch system variable. For additional discussion, see Section 9.2.2.3, “Optimizing Derived Tables and View References”.

A reason to specify TEMPTABLE explicitly is that locks can be released on underlying tables after the temporary table has been created and before it is used to finish processing the statement. This might result in quicker lock release than the MERGE algorithm so that other clients that use the view are not blocked as long.

A view algorithm can be UNDEFINED for three reasons:

  • No ALGORITHM clause is present in the CREATE VIEW statement.

  • The CREATE VIEW statement has an explicit ALGORITHM = UNDEFINED clause.

  • ALGORITHM = MERGE is specified for a view that can be processed only with a temporary table. In this case, MySQL generates a warning and sets the algorithm to UNDEFINED.

As mentioned earlier, MERGE is handled by merging corresponding parts of a view definition into the statement that refers to the view. The following examples briefly illustrate how the MERGE algorithm works. The examples assume that there is a view v_merge that has this definition:

CREATE ALGORITHM = MERGE VIEW v_merge (vc1, vc2) AS
SELECT c1, c2 FROM t WHERE c3 > 100;

Example 1: Suppose that we issue this statement:

SELECT * FROM v_merge;

MySQL handles the statement as follows:

  • v_merge becomes t

  • * becomes vc1, vc2, which corresponds to c1, c2

  • The view WHERE clause is added

The resulting statement to be executed becomes:

SELECT c1, c2 FROM t WHERE c3 > 100;

Example 2: Suppose that we issue this statement:

SELECT * FROM v_merge WHERE vc1 < 100;

This statement is handled similarly to the previous one, except that vc1 < 100 becomes c1 < 100 and the view WHERE clause is added to the statement WHERE clause using an AND connective (and parentheses are added to make sure the parts of the clause are executed with correct precedence). The resulting statement to be executed becomes:

SELECT c1, c2 FROM t WHERE (c3 > 100) AND (c1 < 100);

Effectively, the statement to be executed has a WHERE clause of this form:

WHERE (select WHERE) AND (view WHERE)

If the MERGE algorithm cannot be used, a temporary table must be used instead. Constructs that prevent merging are the same as those that prevent merging in derived tables. Examples are SELECT DISTINCT or LIMIT in the subquery. For details, see Section 9.2.2.3, “Optimizing Derived Tables and View References”.

22.5.3 Updatable and Insertable Views

Some views are updatable and references to them can be used to specify tables to be updated in data change statements. That is, you can use them in statements such as UPDATE, DELETE, or INSERT to update the contents of the underlying table. Derived tables can also be specified in multiple-table UPDATE and DELETE statements, but can only be used for reading data to specify rows to be updated or deleted. Generally, the view references must be updatable, meaning that they may be merged and not materialized. Composite views have more complex rules.

For a view to be updatable, there must be a one-to-one relationship between the rows in the view and the rows in the underlying table. There are also certain other constructs that make a view nonupdatable. To be more specific, a view is not updatable if it contains any of the following:

  • Aggregate functions (SUM(), MIN(), MAX(), COUNT(), and so forth)

  • DISTINCT

  • GROUP BY

  • HAVING

  • UNION or UNION ALL

  • Subquery in the select list

    Before MySQL 5.7.11, subqueries in the select list fail for INSERT, but are okay for UPDATE, DELETE. As of MySQL 5.7.11, that is still true for nondependent subqueries. For dependent subqueries in the select list, no data change statements are permitted.

  • Certain joins (see additional join discussion later in this section)

  • Reference to nonupdatable view in the FROM clause

  • Subquery in the WHERE clause that refers to a table in the FROM clause

  • Refers only to literal values (in this case, there is no underlying table to update)

  • ALGORITHM = TEMPTABLE (use of a temporary table always makes a view nonupdatable)

  • Multiple references to any column of a base table (fails for INSERT, okay for UPDATE, DELETE)

A generated column in a view is considered updatable because it is possible to assign to it. However, if such a column is updated explicitly, the only permitted value is DEFAULT. For information about generated columns, see Section 14.1.18.7, “CREATE TABLE and Generated Columns”.

It is sometimes possible for a multiple-table view to be updatable, assuming that it can be processed with the MERGE algorithm. For this to work, the view must use an inner join (not an outer join or a UNION). Also, only a single table in the view definition can be updated, so the SET clause must name only columns from one of the tables in the view. Views that use UNION ALL are not permitted even though they might be theoretically updatable.

With respect to insertability (being updatable with INSERT statements), an updatable view is insertable if it also satisfies these additional requirements for the view columns:

  • There must be no duplicate view column names.

  • The view must contain all columns in the base table that do not have a default value.

  • The view columns must be simple column references. They must not be expressions, such as these:

    3.14159
    col1 + 3
    UPPER(col2)
    col3 / col4
    (subquery)
    

MySQL sets a flag, called the view updatability flag, at CREATE VIEW time. The flag is set to YES (true) if UPDATE and DELETE (and similar operations) are legal for the view. Otherwise, the flag is set to NO (false). The IS_UPDATABLE column in the INFORMATION_SCHEMA.VIEWS table displays the status of this flag.

If a view is not updatable, statements such UPDATE, DELETE, and INSERT are illegal and are rejected. (Note that even if a view is updatable, it might not be possible to insert into it, as described elsewhere in this section.)

The IS_UPDATABLE flag may be unreliable if a view depends on one or more other views, and one of these underlying views is updated. Regardless of the IS_UPDATABLE value, the server keeps track of the updatability of a view and correctly rejects data change operations to views that are not updatable. If the IS_UPDATABLE value for a view has become inaccurate to due to changes to underlying views, the value can be updated by deleting and recreating the view.

The updatability of views may be affected by the value of the updatable_views_with_limit system variable. See Section 6.1.5, “Server System Variables”.

For the following discussion, suppose that these tables and views exist:

CREATE TABLE t1 (x INTEGER);
CREATE TABLE t2 (c INTEGER);
CREATE VIEW vmat AS SELECT SUM(x) AS s FROM t1;
CREATE VIEW vup AS SELECT * FROM t2;
CREATE VIEW vjoin AS SELECT * FROM vmat JOIN vup ON vmat.s=vup.c;

INSERT, UPDATE, and DELETE statements are permitted as follows:

  • INSERT: The insert table of an INSERT statement may be a view reference that is merged. If the view is a join view, all components of the view must be updatable (not materialized). For a multiple-table updatable view, INSERT can work if it inserts into a single table.

    This statement is invalid because one component of the join view is nonupdatable:

    INSERT INTO vjoin (c) VALUES (1);
    

    This statement is valid; the view contains no materialized components:

    INSERT INTO vup (c) VALUES (1);
    
  • UPDATE: The table or tables to be updated in an UPDATE statement may be view references that are merged. If a view is a join view, at least one component of the view must be updatable (this differs from INSERT).

    In a multiple-table UPDATE statement, the updated table references of the statement must be base tables or updatable view references. Nonupdated table references may be materialized views or derived tables.

    This statement is valid; column c is from the updatable part of the join view:

    UPDATE vjoin SET c=c+1;
    

    This statement is invalid; column x is from the nonupdatable part:

    UPDATE vjoin SET x=x+1;
    

    This statement is valid; the updated table reference of the multiple-table UPDATE is an updatable view (vup):

    UPDATE vup JOIN (SELECT SUM(x) AS s FROM t1) AS dt ON ...
    SET c=c+1;
    

    This statement is invalid; it tries to update a materialized derived table:

    UPDATE vup JOIN (SELECT SUM(x) AS s FROM t1) AS dt ON ...
    SET s=s+1;
    
  • DELETE: The table or tables to be deleted from in a DELETE statement must be merged views. Join views are not allowed (this differs from INSERT and UPDATE).

    This statement is invalid because the view is a join view:

    DELETE vjoin WHERE ...;
    

    This statement is valid because the view is a merged (updatable) view:

    DELETE vup WHERE ...;
    

    This statement is valid because it deletes from a merged (updatable) view:

    DELETE vup FROM vup JOIN (SELECT SUM(x) AS s FROM t1) AS dt ON ...;
    

Additional discussion and examples follow.

Earlier discussion in this section pointed out that a view is not insertable if not all columns are simple column references (for example, if it contains columns that are expressions or composite expressions). Although such a view is not insertable, it can be updatable if you update only columns that are not expressions. Consider this view:

CREATE VIEW v AS SELECT col1, 1 AS col2 FROM t;

This view is not insertable because col2 is an expression. But it is updatable if the update does not try to update col2. This update is permissible:

UPDATE v SET col1 = 0;

This update is not permissible because it attempts to update an expression column:

UPDATE v SET col2 = 0;

If a table contains an AUTO_INCREMENT column, inserting into an insertable view on the table that does not include the AUTO_INCREMENT column does not change the value of LAST_INSERT_ID(), because the side effects of inserting default values into columns not part of the view should not be visible.

22.5.4 The View WITH CHECK OPTION Clause

The WITH CHECK OPTION clause can be given for an updatable view to prevent inserts to rows for which the WHERE clause in the select_statement is not true. It aslo prevents updates to rows for which the WHERE clause is true but the update would cause it to be not true (in other words, it prevents visible rows from being updated to nonvisible rows).

In a WITH CHECK OPTION clause for an updatable view, the LOCAL and CASCADED keywords determine the scope of check testing when the view is defined in terms of another view. When neither keyword is given, the default is CASCADED.

Before MySQL 5.7.6, WITH CHECK OPTION testing works like this:

  • With LOCAL, the view WHERE clause is checked, but no underlying views are checked.

  • With CASCADED, the view WHERE clause is checked, then checking recurses to underlying views, adds WITH CASCADED CHECK OPTION to them (for purposes of the check; their definitions remain unchanged), and applies the same rules.

  • With no check option, the view WHERE clause is not checked, and no underlying views are checked.

As of MySQL 5.7.6, WITH CHECK OPTION testing is standard-compliant (with changed semantics from previously for LOCAL and no check clause):

  • With LOCAL, the view WHERE clause is checked, then checking recurses to underlying views and applies the same rules.

  • With CASCADED, the view WHERE clause is checked, then checking recurses to underlying views, adds WITH CASCADED CHECK OPTION to them (for purposes of the check; their definitions remain unchanged), and applies the same rules.

  • With no check option, the view WHERE clause is not checked, then checking recurses to underlying views, and applies the same rules.

Consider the definitions for the following table and set of views:

CREATE TABLE t1 (a INT);
CREATE VIEW v1 AS SELECT * FROM t1 WHERE a < 2
WITH CHECK OPTION;
CREATE VIEW v2 AS SELECT * FROM v1 WHERE a > 0
WITH LOCAL CHECK OPTION;
CREATE VIEW v3 AS SELECT * FROM v1 WHERE a > 0
WITH CASCADED CHECK OPTION;

Here the v2 and v3 views are defined in terms of another view, v1. Before MySQL 5.7.6, because v2 has a LOCAL check option, inserts are tested only against the v2 check. v3 has a CASCADED check option, so inserts are tested not only against the v3 check, but against those of underlying views. The following statements illustrate these differences:

mysql> INSERT INTO v2 VALUES (2);
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO v3 VALUES (2);
ERROR 1369 (HY000): CHECK OPTION failed 'test.v3'

As of MySQL 5.7.6, the semantics for LOCAL differ from previously: Inserts for v2 are checked against its LOCAL check option, then (unlike before 5.7.6), the check recurses to v1 and the rules are applied again. The rules for v1 cause a check failure. The check for v3 fails as before:

mysql> INSERT INTO v2 VALUES (2);
ERROR 1369 (HY000): CHECK OPTION failed 'test.v2'
mysql> INSERT INTO v3 VALUES (2);
ERROR 1369 (HY000): CHECK OPTION failed 'test.v3'

22.5.5 View Metadata

Metadata about views can be obtained as follows:

22.6 Access Control for Stored Programs and Views

Stored programs and views are defined prior to use and, when referenced, execute within a security context that determines their privileges. These privileges are controlled by their DEFINER attribute, and, if there is one, their SQL SECURITY characteristic.

All stored programs (procedures, functions, triggers, and events) and views can have a DEFINER attribute that names a MySQL account. If the DEFINER attribute is omitted from a stored program or view definition, the default account is the user who creates the object.

In addition, stored routines (procedures and functions) and views can have a SQL SECURITY characteristic with a value of DEFINER or INVOKER to specify whether the object executes in definer or invoker context. If the SQL SECURITY characteristic is omitted, the default is definer context.

Triggers and events have no SQL SECURITY characteristic and always execute in definer context. The server invokes these objects automatically as necessary, so there is no invoking user.

Definer and invoker security contexts differ as follows:

  • A stored program or view that executes in definer security context executes with the privileges of the account named by its DEFINER attribute. These privileges may be entirely different from those of the invoking user. The invoker must have appropriate privileges to reference the object (for example, EXECUTE to call a stored procedure or SELECT to select from a view), but when the object executes, the invoker's privileges are ignored and only the DEFINER account privileges matter. If this account has few privileges, the object is correspondingly limited in the operations it can perform. If the DEFINER account is highly privileged (such as a root account), the object can perform powerful operations no matter who invokes it.

  • A stored routine or view that executes in invoker security context can perform only operations for which the invoker has privileges. The DEFINER attribute can be specified but has no effect for objects that execute in invoker context.

Consider the following stored procedure:

CREATE DEFINER = 'admin'@'localhost' PROCEDURE p1()
SQL SECURITY DEFINER
BEGIN
  UPDATE t1 SET counter = counter + 1;
END;

Any user who has the EXECUTE privilege for p1 can invoke it with a CALL statement. However, when p1 executes, it does so in DEFINER security context and thus executes with the privileges of 'admin'@'localhost', the account named in the DEFINER attribute. This account must have the EXECUTE privilege for p1 as well as the UPDATE privilege for the table t1. Otherwise, the procedure fails.

Now consider this stored procedure, which is identical to p1 except that its SQL SECURITY characteristic is INVOKER:

CREATE DEFINER = 'admin'@'localhost' PROCEDURE p2()
SQL SECURITY INVOKER
BEGIN
  UPDATE t1 SET counter = counter + 1;
END;

p2, unlike p1, executes in INVOKER security context. The DEFINER attribute is irrelevant and p2 executes with the privileges of the invoking user. p2 fails if the invoker lacks the EXECUTE privilege for p2 or the UPDATE privilege for the table t1.

MySQL uses the following rules to control which accounts a user can specify in an object DEFINER attribute:

  • You can specify a DEFINER value other than your own account only if you have the SUPER privilege.

  • If you do not have the SUPER privilege, the only legal user value is your own account, either specified literally or by using CURRENT_USER. You cannot set the definer to some other account.

To minimize the risk potential for stored program and view creation and use, follow these guidelines:

  • For a stored routine or view, use SQL SECURITY INVOKER in the object definition when possible so that it can be used only by users with permissions appropriate for the operations performed by the object.

  • If you create definer-context stored programs or views while using an account that has the SUPER privilege, specify an explicit DEFINER attribute that names an account possessing only the privileges required for the operations performed by the object. Specify a highly privileged DEFINER account only when absolutely necessary.

  • Administrators can prevent users from specifying highly privileged DEFINER accounts by not granting them the SUPER privilege.

  • Definer-context objects should be written keeping in mind that they may be able to access data for which the invoking user has no privileges. In some cases, you can prevent reference to these objects by not granting unauthorized users particular privileges:

    • A stored procedure or function cannot be referenced by a user who does not have the EXECUTE privilege for it.

    • A view cannot be referenced by a user who does not have the appropriate privilege for it (SELECT to select from it, INSERT to insert into it, and so forth).

    However, no such control exists for triggers because users do not reference them directly. A trigger always executes in DEFINER context and is activated by access to the table with which it is associated, even ordinary table accesses by users with no special privileges. If the DEFINER account is highly privileged, the trigger can perform sensitive or dangerous operations. This remains true if the SUPER and TRIGGER privileges needed to create the trigger are revoked from the account of the user who created it. Administrators should be especially careful about granting users that combination of privileges.

22.7 Binary Logging of Stored Programs

The binary log contains information about SQL statements that modify database contents. This information is stored in the form of events that describe the modifications. The binary log has two important purposes:

  • For replication, the binary log is used on master replication servers as a record of the statements to be sent to slave servers. The master server sends the events contained in its binary log to its slaves, which execute those events to make the same data changes that were made on the master. See Section 18.2, “Replication Implementation”.

  • Certain data recovery operations require use of the binary log. After a backup file has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup. See Section 8.3.2, “Using Backups for Recovery”.

However, if logging occurs at the statement level, there are certain binary logging issues with respect to stored programs (stored procedures and functions, triggers, and events):

  • In some cases, a statement might affect different sets of rows on master and slave.

  • Replicated statements executed on a slave are processed by the slave SQL thread, which has full privileges. It is possible for a procedure to follow different execution paths on master and slave servers, so a user can write a routine containing a dangerous statement that will execute only on the slave where it is processed by a thread that has full privileges.

  • If a stored program that modifies data is nondeterministic, it is not repeatable. This can result in different data on master and slave, or cause restored data to differ from the original data.

This section describes how MySQL handles binary logging for stored programs. It states the current conditions that the implementation places on the use of stored programs, and what you can do to avoid logging problems. It also provides additional information about the reasons for these conditions.

In general, the issues described here result when binary logging occurs at the SQL statement level. If you use row-based binary logging, the log contains changes made to individual rows as a result of executing SQL statements. When routines or triggers execute, row changes are logged, not the statements that make the changes. For stored procedures, this means that the CALL statement is not logged. For stored functions, row changes made within the function are logged, not the function invocation. For triggers, row changes made by the trigger are logged. On the slave side, only the row changes are seen, not the stored program invocation. For general information about row-based logging, see Section 18.2.1, “Replication Formats”.

Unless noted otherwise, the remarks here assume that you have enabled binary logging by starting the server with the --log-bin option. (See Section 6.4.4, “The Binary Log”.) If the binary log is not enabled, replication is not possible, nor is the binary log available for data recovery.

The conditions on the use of stored functions in MySQL can be summarized as follows. These conditions do not apply to stored procedures or Event Scheduler events and they do not apply unless binary logging is enabled.

  • To create or alter a stored function, you must have the SUPER privilege, in addition to the CREATE ROUTINE or ALTER ROUTINE privilege that is normally required. (Depending on the DEFINER value in the function definition, SUPER might be required regardless of whether binary logging is enabled. See Section 14.1.16, “CREATE PROCEDURE and CREATE FUNCTION Syntax”.)

  • When you create a stored function, you must declare either that it is deterministic or that it does not modify data. Otherwise, it may be unsafe for data recovery or replication.

    By default, for a CREATE FUNCTION statement to be accepted, at least one of DETERMINISTIC, NO SQL, or READS SQL DATA must be specified explicitly. Otherwise an error occurs:

    ERROR 1418 (HY000): This function has none of DETERMINISTIC, NO SQL,
    or READS SQL DATA in its declaration and binary logging is enabled
    (you *might* want to use the less safe log_bin_trust_function_creators
    variable)
    

    This function is deterministic (and does not modify data), so it is safe:

    CREATE FUNCTION f1(i INT)
    RETURNS INT
    DETERMINISTIC
    READS SQL DATA
    BEGIN
      RETURN i;
    END;
    

    This function uses UUID(), which is not deterministic, so the function also is not deterministic and is not safe:

    CREATE FUNCTION f2()
    RETURNS CHAR(36) CHARACTER SET utf8
    BEGIN
      RETURN UUID();
    END;
    

    This function modifies data, so it may not be safe:

    CREATE FUNCTION f3(p_id INT)
    RETURNS INT
    BEGIN
      UPDATE t SET modtime = NOW() WHERE id = p_id;
      RETURN ROW_COUNT();
    END;
    

    Assessment of the nature of a function is based on the honesty of the creator: MySQL does not check that a function declared DETERMINISTIC is free of statements that produce nondeterministic results.

  • Although it is possible to create a deterministic stored function without specifying DETERMINISTIC, you cannot execute this function using statement-based binary logging. To execute such a function, you must use row-based or mixed binary logging. Alternatively, if you explicitly specify DETERMINISTIC in the function definition, you can use any kind of logging, including statement-based logging.

  • To relax the preceding conditions on function creation (that you must have the SUPER privilege and that a function must be declared deterministic or to not modify data), set the global log_bin_trust_function_creators system variable to 1. By default, this variable has a value of 0, but you can change it like this:

    mysql> SET GLOBAL log_bin_trust_function_creators = 1;
    

    You can also set this variable by using the --log-bin-trust-function-creators=1 option when starting the server.

    If binary logging is not enabled, log_bin_trust_function_creators does not apply. SUPER is not required for function creation unless, as described previously, the DEFINER value in the function definition requires it.

  • For information about built-in functions that may be unsafe for replication (and thus cause stored functions that use them to be unsafe as well), see Section 18.4.1, “Replication Features and Issues”.

Triggers are similar to stored functions, so the preceding remarks regarding functions also apply to triggers with the following exception: CREATE TRIGGER does not have an optional DETERMINISTIC characteristic, so triggers are assumed to be always deterministic. However, this assumption might be invalid in some cases. For example, the UUID() function is nondeterministic (and does not replicate). Be careful about using such functions in triggers.

Triggers can update tables, so error messages similar to those for stored functions occur with CREATE TRIGGER if you do not have the required privileges. On the slave side, the slave uses the trigger DEFINER attribute to determine which user is considered to be the creator of the trigger.

The rest of this section provides additional detail about the logging implementation and its implications. You need not read it unless you are interested in the background on the rationale for the current logging-related conditions on stored routine use. This discussion applies only for statement-based logging, and not for row-based logging, with the exception of the first item: CREATE and DROP statements are logged as statements regardless of the logging mode.

  • The server writes CREATE EVENT, CREATE PROCEDURE, CREATE FUNCTION, ALTER EVENT, ALTER PROCEDURE, ALTER FUNCTION, DROP EVENT, DROP PROCEDURE, and DROP FUNCTION statements to the binary log.

  • A stored function invocation is logged as a SELECT statement if the function changes data and occurs within a statement that would not otherwise be logged. This prevents nonreplication of data changes that result from use of stored functions in nonlogged statements. For example, SELECT statements are not written to the binary log, but a SELECT might invoke a stored function that makes changes. To handle this, a SELECT func_name() statement is written to the binary log when the given function makes a change. Suppose that the following statements are executed on the master:

    CREATE FUNCTION f1(a INT) RETURNS INT
    BEGIN
      IF (a < 3) THEN
        INSERT INTO t2 VALUES (a);
      END IF;
      RETURN 0;
    END;
    
    CREATE TABLE t1 (a INT);
    INSERT INTO t1 VALUES (1),(2),(3);
    
    SELECT f1(a) FROM t1;
    

    When the SELECT statement executes, the function f1() is invoked three times. Two of those invocations insert a row, and MySQL logs a SELECT statement for each of them. That is, MySQL writes the following statements to the binary log:

    SELECT f1(1);
    SELECT f1(2);
    

    The server also logs a SELECT statement for a stored function invocation when the function invokes a stored procedure that causes an error. In this case, the server writes the SELECT statement to the log along with the expected error code. On the slave, if the same error occurs, that is the expected result and replication continues. Otherwise, replication stops.

  • Logging stored function invocations rather than the statements executed by a function has a security implication for replication, which arises from two factors:

    • It is possible for a function to follow different execution paths on master and slave servers.

    • Statements executed on a slave are processed by the slave SQL thread which has full privileges.

    The implication is that although a user must have the CREATE ROUTINE privilege to create a function, the user can write a function containing a dangerous statement that will execute only on the slave where it is processed by a thread that has full privileges. For example, if the master and slave servers have server ID values of 1 and 2, respectively, a user on the master server could create and invoke an unsafe function unsafe_func() as follows:

    mysql> delimiter //
    mysql> CREATE FUNCTION unsafe_func () RETURNS INT
        -> BEGIN
        ->   IF @@server_id=2 THEN dangerous_statement; END IF;
        ->   RETURN 1;
        -> END;
        -> //
    mysql> delimiter ;
    mysql> INSERT INTO t VALUES(unsafe_func());
    

    The CREATE FUNCTION and INSERT statements are written to the binary log, so the slave will execute them. Because the slave SQL thread has full privileges, it will execute the dangerous statement. Thus, the function invocation has different effects on the master and slave and is not replication-safe.

    To guard against this danger for servers that have binary logging enabled, stored function creators must have the SUPER privilege, in addition to the usual CREATE ROUTINE privilege that is required. Similarly, to use ALTER FUNCTION, you must have the SUPER privilege in addition to the ALTER ROUTINE privilege. Without the SUPER privilege, an error will occur:

    ERROR 1419 (HY000): You do not have the SUPER privilege and
    binary logging is enabled (you *might* want to use the less safe
    log_bin_trust_function_creators variable)
    

    If you do not want to require function creators to have the SUPER privilege (for example, if all users with the CREATE ROUTINE privilege on your system are experienced application developers), set the global log_bin_trust_function_creators system variable to 1. You can also set this variable by using the --log-bin-trust-function-creators=1 option when starting the server. If binary logging is not enabled, log_bin_trust_function_creators does not apply. SUPER is not required for function creation unless, as described previously, the DEFINER value in the function definition requires it.

  • If a function that performs updates is nondeterministic, it is not repeatable. This can have two undesirable effects:

    • It will make a slave different from the master.

    • Restored data will be different from the original data.

    To deal with these problems, MySQL enforces the following requirement: On a master server, creation and alteration of a function is refused unless you declare the function to be deterministic or to not modify data. Two sets of function characteristics apply here:

    • The DETERMINISTIC and NOT DETERMINISTIC characteristics indicate whether a function always produces the same result for given inputs. The default is NOT DETERMINISTIC if neither characteristic is given. To declare that a function is deterministic, you must specify DETERMINISTIC explicitly.

    • The CONTAINS SQL, NO SQL, READS SQL DATA, and MODIFIES SQL DATA characteristics provide information about whether the function reads or writes data. Either NO SQL or READS SQL DATA indicates that a function does not change data, but you must specify one of these explicitly because the default is CONTAINS SQL if no characteristic is given.

    By default, for a CREATE FUNCTION statement to be accepted, at least one of DETERMINISTIC, NO SQL, or READS SQL DATA must be specified explicitly. Otherwise an error occurs:

    ERROR 1418 (HY000): This function has none of DETERMINISTIC, NO SQL,
    or READS SQL DATA in its declaration and binary logging is enabled
    (you *might* want to use the less safe log_bin_trust_function_creators
    variable)
    

    If you set log_bin_trust_function_creators to 1, the requirement that functions be deterministic or not modify data is dropped.

  • Stored procedure calls are logged at the statement level rather than at the CALL level. That is, the server does not log the CALL statement, it logs those statements within the procedure that actually execute. As a result, the same changes that occur on the master will be observed on slave servers. This prevents problems that could result from a procedure having different execution paths on different machines.

    In general, statements executed within a stored procedure are written to the binary log using the same rules that would apply were the statements to be executed in standalone fashion. Some special care is taken when logging procedure statements because statement execution within procedures is not quite the same as in nonprocedure context:

    • A statement to be logged might contain references to local procedure variables. These variables do not exist outside of stored procedure context, so a statement that refers to such a variable cannot be logged literally. Instead, each reference to a local variable is replaced by this construct for logging purposes:

      NAME_CONST(var_name, var_value)
      

      var_name is the local variable name, and var_value is a constant indicating the value that the variable has at the time the statement is logged. NAME_CONST() has a value of var_value, and a name of var_name. Thus, if you invoke this function directly, you get a result like this:

      mysql> SELECT NAME_CONST('myname', 14);
      +--------+
      | myname |
      +--------+
      |     14 |
      +--------+
      

      NAME_CONST() enables a logged standalone statement to be executed on a slave with the same effect as the original statement that was executed on the master within a stored procedure.

      The use of NAME_CONST() can result in a problem for CREATE TABLE ... SELECT statements when the source column expressions refer to local variables. Converting these references to NAME_CONST() expressions can result in column names that are different on the master and slave servers, or names that are too long to be legal column identifiers. A workaround is to supply aliases for columns that refer to local variables. Consider this statement when myvar has a value of 1:

      CREATE TABLE t1 SELECT myvar;
      

      That will be rewritten as follows:

      CREATE TABLE t1 SELECT NAME_CONST(myvar, 1);
      

      To ensure that the master and slave tables have the same column names, write the statement like this:

      CREATE TABLE t1 SELECT myvar AS myvar;
      

      The rewritten statement becomes:

      CREATE TABLE t1 SELECT NAME_CONST(myvar, 1) AS myvar;
      
    • A statement to be logged might contain references to user-defined variables. To handle this, MySQL writes a SET statement to the binary log to make sure that the variable exists on the slave with the same value as on the master. For example, if a statement refers to a variable @my_var, that statement will be preceded in the binary log by the following statement, where value is the value of @my_var on the master:

      SET @my_var = value;
      
    • Procedure calls can occur within a committed or rolled-back transaction. Transactional context is accounted for so that the transactional aspects of procedure execution are replicated correctly. That is, the server logs those statements within the procedure that actually execute and modify data, and also logs BEGIN, COMMIT, and ROLLBACK statements as necessary. For example, if a procedure updates only transactional tables and is executed within a transaction that is rolled back, those updates are not logged. If the procedure occurs within a committed transaction, BEGIN and COMMIT statements are logged with the updates. For a procedure that executes within a rolled-back transaction, its statements are logged using the same rules that would apply if the statements were executed in standalone fashion:

      • Updates to transactional tables are not logged.

      • Updates to nontransactional tables are logged because rollback does not cancel them.

      • Updates to a mix of transactional and nontransactional tables are logged surrounded by BEGIN and ROLLBACK so that slaves will make the same changes and rollbacks as on the master.

  • A stored procedure call is not written to the binary log at the statement level if the procedure is invoked from within a stored function. In that case, the only thing logged is the statement that invokes the function (if it occurs within a statement that is logged) or a DO statement (if it occurs within a statement that is not logged). For this reason, care should be exercised in the use of stored functions that invoke a procedure, even if the procedure is otherwise safe in itself.