Chapter 6. InnoDB INFORMATION_SCHEMA Tables

The tables INNODB_CMP and INNODB_CMP_RESET contain status information on the operations related to compressed tables, which are covered in Chapter 3, InnoDB Data Compression. The compressed page size is in the column PAGE_SIZE.

These two tables have identical contents, but reading from INNODB_CMP_RESET resets the statistics on compression and uncompression operations. For example, if you archived the output of INNODB_CMP_RESET every 60 minutes, it would show the hourly statistics. If you never read INNODB_CMP_RESET and monitored the output of INNODB_CMP instead, it would show the cumulated statistics since InnoDB was started.

You may consider the tables INNODB_CMPMEM and INNODB_CMPMEM_RESET as the status information on the compressed pages that reside in the buffer pool. Please consult Chapter 3, InnoDB Data Compression for further information on compressed tables and the use of the buffer pool. The tables INNODB_CMP and INNODB_CMP_RESET should provide more useful statistics on compression.

The InnoDB Plugin uses a so-called “buddy allocator” system to manage memory allocated to pages of various sizes, from 1KB to 16KB. Each row of the two tables described here corresponds to a single page size.

These two tables have identical contents, but reading from INNODB_CMPMEM_RESET resets the statistics on relocation operations. For example, if every 60 minutes you archived the output of INNODB_CMPMEM_RESET, it would show the hourly statistics. If you never read INNODB_CMPMEM_RESET and monitored the output of INNODB_CMPMEM instead, it would show the cumulated statistics since InnoDB was started.

Contains information about every transaction currently executing inside InnoDB, including whether the transaction is waiting for a lock, when the transaction started, and the particular SQL statement the transaction is executing.

Each transaction in InnoDB that is waiting for another transaction to release a lock (INNODB_TRX.TRX_STATE='LOCK WAIT') is blocked by exactly one “blocking lock request”. That blocking lock request is for a row or table lock held by another transaction in an incompatible mode. The waiting or blocked transaction cannot proceed until the other transaction commits or rolls back, thereby releasing the requested lock. For every blocked transaction, INNODB_LOCKS contains one row that describes each lock the transaction has requested, and for which it is waiting. INNODB_LOCKS also contains one row for each lock that is blocking another transaction, whatever the state of the transaction that holds the lock ('RUNNING', 'LOCK WAIT', 'ROLLING BACK' or 'COMMITTING'). The lock that is blocking a transaction is always held in a mode (read vs. write, shared vs. exclusive) incompatible with the mode of requested lock.

Using this table, you can tell which transactions are waiting for a given lock, or for which lock a given transaction is waiting. This table contains one or more rows for each blocked transaction, indicating the lock it has requested and the lock(s) that is (are) blocking that request. The REQUESTED_LOCK_ID refers to the lock that a transaction is requesting, and the BLOCKING_LOCK_ID refers to the lock (held by another transaction) that is preventing the first transaction from proceeding. For any given blocked transaction, all rows in INNODB_LOCK_WAITS have the same value for REQUESTED_LOCK_ID and different values for BLOCKING_LOCK_ID.

When a transaction updates a row in a table, or locks it with SELECT FOR UPDATE, InnoDB establishes a list or queue of locks on that row. Similarly, InnoDB maintains a list of locks on a table for table-level locks transactions hold. If a second transaction wants to update a row or lock a table already locked by a prior transaction in an incompatible mode, InnoDB adds a lock request for the row to the corresponding queue. For a lock to be acquired by a transaction, all incompatible lock requests previously entered into the lock queue for that row or table must be removed (the transactions holding or requesting those locks either commit or rollback).

A transaction may have any number of lock requests for different rows or tables. At any given time, a transaction may be requesting a lock that is held by another transaction, in which case it is blocked by that other transaction. The requesting transaction must wait for the transaction that holds the blocking lock to commit or rollback. If a transaction is not waiting for a a lock, it is in the 'RUNNING' state. If a transaction is waiting for a lock, it is in the 'LOCK WAIT' state.

The table INNODB_LOCKS holds one or more row for each 'LOCK WAIT' transaction, indicating the lock request(s) that is (are) preventing its progress. This table also contains one row describing each lock in a queue of locks pending for a given row or table. The table INNODB_LOCK_WAITS shows which locks already held by a transaction are blocking locks requested by other transactions.

The data exposed by the transaction and locking tables represent a glimpse into fast-changing data. This is not like other (user) tables, where the data only changes 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 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.

As just described, while the transaction and locking data is correct and consistent when these INFORMATION_SCHEMA tables are populated, the underlying data changes so fast that similar glimpses at other, similarly fast-changing data, may not be in sync. Thus, you should be careful in comparing the data in the InnoDB transaction and locking tables with that in the MySQL table PROCESSLIST . 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 be different from the one in PROCESSLIST.INFO. The query in INNODB_TRX is always consistent with the rest of INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS when the data comes from the same snapshot.