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MySQL 5.5 Reference Manual  /  ...  /  Data Persistence and Consistency for InnoDB Transaction and Locking Tables Data Persistence and Consistency for InnoDB Transaction and Locking Tables

The data exposed by the transaction and locking tables (INNODB_TRX, INNODB_LOCKS, and INNODB_LOCK_WAITS) represent a glimpse into fast-changing data. This is not like other (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.

For performance reasons, and to minimize the chance of misleading JOINs between the InnoDB transaction and locking INFORMATION_SCHEMA tables, InnoDB collects the required transaction and locking information into an intermediate buffer whenever a SELECT on any of the tables is issued. This buffer is refreshed only if more than 0.1 seconds has elapsed since the last time the buffer was read. The data needed to fill the three tables is fetched atomically and consistently and is saved in this global internal buffer, forming a point-in-time snapshot. If multiple table accesses occur within 0.1 seconds (as they almost certainly do when MySQL processes a join among these tables), then the same snapshot is used to satisfy the query.

A correct result is returned when you JOIN any of these tables together in a single query, because the data for the three tables comes from the same snapshot. Because the buffer is not refreshed with every query of any of these tables, if you issue separate queries against these tables within a tenth of a second, the results are the same from query to query. On the other hand, two separate queries of the same or different tables issued more than a tenth of a second apart may see different results, since the data come from different snapshots.

Because InnoDB must temporarily stall while the transaction and locking data is collected, too frequent queries of these tables can negatively impact performance as seen by other users.

As these tables contain sensitive information (at least INNODB_LOCKS.LOCK_DATA and INNODB_TRX.TRX_QUERY), for security reasons, only the users with the PROCESS privilege are allowed to SELECT from them. Potential Inconsistency with PROCESSLIST Data

As described in Section, “Data Persistence and Consistency for InnoDB Transaction and Locking Tables”, the data that fills the InnoDB transaction and locking tables (INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS) is fetched automatically and saved to an intermediate buffer that provides a point-in-time snapshot. The data across all three tables is consistent when queried from the same snapshot. However, the underlying data changes so fast that similar glimpses at other, similarly fast-changing data, may not be in synchrony. Thus, you should be careful when comparing data in the InnoDB transaction and locking tables with data in the PROCESSLIST table. The data from the PROCESSLIST table does not come from the same snapshot as the data about locking and transactions. Even if you issue a single SELECT (joining INNODB_TRX and PROCESSLIST, for example), the content of those tables is generally not consistent. INNODB_TRX may reference rows that are not present in PROCESSLIST or the currently executing SQL query of a transaction, shown in INNODB_TRX.TRX_QUERY may differ from the one in PROCESSLIST.INFO.

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