17.15.2.3 Persistence and Consistency of InnoDB Transaction and Locking Information

The data exposed by the transaction and locking tables (INFORMATION_SCHEMA INNODB_TRX table, Performance Schema data_locks and data_lock_waits tables) represents a glimpse into fast-changing data. This is not like user tables, where the data changes only when application-initiated updates occur. The underlying data is internal system-managed data, and can change very quickly:

• Data might not be consistent between the INNODB_TRX, data_locks, and data_lock_waits tables.

  The data_locks and data_lock_waits tables expose live data from the InnoDB storage engine, to provide lock information about the transactions in the INNODB_TRX table. Data retrieved from the lock tables exists when the SELECT is executed, but might be gone or changed by the time the query result is consumed by the client.

  Joining data_locks with data_lock_waits can show rows in data_lock_waits that identify a parent row in data_locks that no longer exists or does not exist yet.

• Data in the transaction and locking tables might not be consistent with data in the INFORMATION_SCHEMA PROCESSLIST table or Performance Schema threads table.

  For example, you should be careful when comparing data in the InnoDB transaction and locking tables with data in the PROCESSLIST table. Even if you issue a single SELECT (joining INNODB_TRX and PROCESSLIST, for example), the content of those tables is generally not consistent. It is possible for INNODB_TRX to reference rows that are not present in PROCESSLIST or for the currently executing SQL query of a transaction shown in INNODB_TRX.TRX_QUERY to differ from the one in PROCESSLIST.INFO.