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21.9.7 Performance Schema Transaction Tables

As of MySQL 5.7.3, the Performance Schema instruments transactions. Within the event hierarchy, wait events nest within stage events, which nest within statement events, which nest within transaction events.

These tables store transaction events:

The following sections describe those tables. There are also summary tables that aggregate information about transaction events; see Section, “Transaction Summary Tables”.

Transaction Event Configuration

To enable collection of transaction events, enable the relevant instruments and consumers.

The setup_instruments table contains an instrument named transaction. This instrument is disabled by default:

mysql> SELECT * FROM setup_instruments WHERE NAME = 'transaction';
| NAME        | ENABLED | TIMED |
| transaction | NO      | NO    |

To enable collection of transaction events, including timing information, do this:

mysql> UPDATE setup_instruments SET ENABLED = 'YES', TIMED = 'YES'
    -> WHERE NAME = 'transaction';

The setup_consumers table contains consumer values with names corresponding to the current and recent transaction event table names. These consumers may be used to filter collection of transaction events:

mysql> SELECT * FROM setup_consumers WHERE NAME LIKE '%transactions%';
| NAME                             | ENABLED |
| events_transactions_current      | NO      |
| events_transactions_history      | NO      |
| events_transactions_history_long | NO      |

To enable all transaction consumers, do this:

mysql> UPDATE setup_consumers SET ENABLED = 'YES'
    -> WHERE NAME LIKE '%transactions%';

To enable collection of transaction events only for specific transaction event tables, enable the corresponding transaction consumers.

The setup_timers table contains a row with a NAME value of transaction that indicates the unit for transaction event timing. The default unit is NANOSECOND.

mysql> SELECT * FROM setup_timers WHERE NAME = 'transaction';
| NAME        | TIMER_NAME |
| transaction | NANOSECOND |

To change the timing unit, modify the TIMER_NAME value:

    -> WHERE NAME = 'transaction';

For additional information about configuring event collection, see Section 21.2, “Performance Schema Configuration”.

Transaction Boundaries

In MySQL Server, transactions start explicitly with these statements:


Transactions also start implicitly. For example, when the autocommit system variable is enabled, the start of each statement starts a new transaction.

When autocommit is disabled, the first statement following a committed transaction marks the start of a new transaction. Subsequent statements are part of the transaction until it is committed.

Transactions explicitly end with these statements:


Transactions also end implicitly, by execution of DDL statements, locking statements, and server administration statements.

In the following discussion, references to START TRANSACTION also apply to BEGIN, XA START, and XA BEGIN. Similarly, references to COMMIT and ROLLBACK apply to XA COMMIT and XA ROLLBACK, respectively.

The Performance Schema defines transaction boundaries similarly to that of the server. The start and end of a transaction event closely match the corresponding state transitions in the server:

  • For an explicitly started transaction, the transaction event starts during processing of the START TRANSACTION statement.

  • For an implicitly started transaction, the transaction event starts on the first statement that uses a transactional engine after the previous transaction has ended.

  • For any transaction, whether explicitly or implicitly ended, the transaction event ends when the server transitions out of the active transaction state during the processing of COMMIT or ROLLBACK.

There are subtle implications to this approach:

  • Transaction events in the Performance Schema do not fully include the statement events associated with the corresponding START TRANSACTION, COMMIT, or ROLLBACK statements. There is a trivial amount of timing overlap between the transaction event and these statements.

  • Statements that work with nontransactional engines have no effect on the transaction state of the connection. For implicit transactions, the transaction event begins with the first statement that uses a transactional engine. This means that statements operating exclusively on nontransactional tables are ignored, even following START TRANSACTION.

To illustrate, consider the following scenario:

1. SET autocommit = OFF;
3. START TRANSACTION;                       -- Transaction 1 START
4. INSERT INTO t1 VALUES (1), (2), (3);
5. CREATE TABLE t2 (a INT) ENGINE = MyISAM; -- Transaction 1 COMMIT
                                            -- (implicit; DDL forces commit)
6. INSERT INTO t2 VALUES (1), (2), (3);     -- Update nontransactional table
7. UPDATE t2 SET a = a + 1;                 -- ... and again
8. INSERT INTO t1 VALUES (4), (5), (6);     -- Write to transactional table
                                            -- Transaction 2 START (implicit)
9. COMMIT;                                  -- Transaction 2 COMMIT

From the perspective of the server, Transaction 1 ends when table t2 is created. Transaction 2 does not start until a transactional table is accessed, despite the intervening updates to nontransactional tables.

From the perspective of the Performance Schema, Transaction 2 starts when the server transitions into an active transaction state. Statements 6 and 7 are not included within the boundaries of Transaction 2, which is consistent with how the server writes transactions to the binary log.

Transaction Instrumentation

Three attributes define transactions:

  • Access mode (read only, read write)

  • Isolation level (SERIALIZABLE, REPEATABLE READ, and so forth)

  • Implicit (autocommit enabled) or explicit (autocommit disabled)

To reduce complexity of the transaction instrumentation and to ensure that the collected transaction data provides complete, meaningful results, all transactions are instrumented independently of access mode, isolation level, or autocommit mode.

To selectively examine transaction history, use the attribute columns in the transaction event tables: ACCESS_MODE, ISOLATION_LEVEL, and AUTOCOMMIT.

The cost of transaction instrumentation can be reduced various ways, such as enabling or disabling transaction instrumentation according to user, account, host, or thread (client connection).

Transactions and Nested Events

The parent of a transaction event is the event that initiated the transaction. For an explicitly started transaction, this includes the START TRANSACTION and COMMIT AND CHAIN statements. For an implicitly started transaction, it is the first statement that uses a transactional engine after the previous transaction ends.

In general, a transaction is the top-level parent to all events initiated during the transaction, including statements that explicitly end the transaction such as COMMIT and ROLLBACK. Exceptions are statements that implicitly end a transaction, such as DDL statements, in which case the current transaction must be committed before the new statement is executed.

Transactions and Stored Programs

Transactions and stored program events are related as follows:

  • Stored Procedures

    Stored procedures operate independently of transactions. A stored procedure can be started within a transaction, and a transaction can be started or ended from within a stored procedure. If called from within a transaction, a stored procedure can execute statements that force a commit of the parent transaction and then start a new transaction.

    If a stored procedure is started within a transaction, that transaction is the parent of the stored procedure event.

    If a transaction is started by a stored procedure, the stored procedure is the parent of the transaction event.

  • Stored Functions

    Stored functions are restricted from causing an explicit or implicit commit or rollback. Stored function events can reside within a parent transaction event.

  • Triggers

    Triggers activate as part of a statement that accesses the table with which it is associated, so the parent of a trigger event is always the statement that activates it.

    Triggers cannot issue statements that cause an explicit or implicit commit or rollback of a transaction.

  • Scheduled Events

    The execution of the statements in the body of a scheduled event takes place in a new connection. Nesting of a scheduled event within a parent transaction is not applicable.

Transactions and Savepoints

Savepoint statements are recorded as separate statement events. Transaction events include separate counters for SAVEPOINT, ROLLBACK TO SAVEPOINT, and RELEASE SAVEPOINT statements issued during the transaction.

Transactions and Errors

Errors and warnings that occur within a transaction are recorded in statement events, but not in the corresponding transaction event. This includes transaction-specific errors and warnings, such as a rollback on a nontransactional table or GTID consistency errors.

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