14.7.2.1 Transaction Isolation Levels

Transaction isolation is one of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.

InnoDB offers all four transaction isolation levels described by the SQL:1992 standard: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. The default isolation level for InnoDB is REPEATABLE READ.

A user can change the isolation level for a single session or for all subsequent connections with the SET TRANSACTION statement. To set the server's default isolation level for all connections, use the --transaction-isolation option on the command line or in an option file. For detailed information about isolation levels and level-setting syntax, see Section 13.3.6, “SET TRANSACTION Statement”.

InnoDB supports each of the transaction isolation levels described here using different locking strategies. You can enforce a high degree of consistency with the default REPEATABLE READ level, for operations on crucial data where ACID compliance is important. Or you can relax the consistency rules with READ COMMITTED or even READ UNCOMMITTED, in situations such as bulk reporting where precise consistency and repeatable results are less important than minimizing the amount of overhead for locking. SERIALIZABLE enforces even stricter rules than REPEATABLE READ, and is used mainly in specialized situations, such as with XA transactions and for troubleshooting issues with concurrency and deadlocks.

The following list describes how MySQL supports the different transaction levels. The list goes from the most commonly used level to the least used.

• REPEATABLE READ

  This is the default isolation level for InnoDB. Consistent reads within the same transaction read the snapshot established by the first read. This means that if you issue several plain (nonlocking) SELECT statements within the same transaction, these SELECT statements are consistent also with respect to each other. See Section 14.7.2.3, “Consistent Nonlocking Reads”.

  For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), UPDATE, and DELETE statements, locking depends on whether the statement uses a unique index with a unique search condition or a range-type search condition.

  • For a unique index with a unique search condition, InnoDB locks only the index record found, not the gap before it.

  • For other search conditions, InnoDB locks the index range scanned, using gap locks or next-key locks to block insertions by other sessions into the gaps covered by the range. For information about gap locks and next-key locks, see Section 14.7.1, “InnoDB Locking”.

  It is not recommended to mix locking statements (UPDATE, INSERT, DELETE, or SELECT ... FOR ...) with non-locking SELECT statements in a single REPEATABLE READ transaction, because typically in such cases you want SERIALIZABLE instead. This is because a non-locking SELECT statement presents the state of the database from a read-view which consists of transactions committed before the read-view was created and before the current transaction's own writes, while the locking statements see and modify the most recent state of the database to use locking. In general, these two different table states are inconsistent with each other and difficult to parse.

• READ COMMITTED

  Each consistent read, even within the same transaction, sets and reads its own fresh snapshot. For information about consistent reads, see Section 14.7.2.3, “Consistent Nonlocking Reads”.

  For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), UPDATE statements, and DELETE statements, InnoDB locks only index records, not the gaps before them, and thus permits the free insertion of new records next to locked records. Gap locking is only used for foreign-key constraint checking and duplicate-key checking.

  Because gap locking is disabled, phantom row problems may occur, as other sessions can insert new rows into the gaps. For information about phantom rows, see Section 14.7.4, “Phantom Rows”.

  Only row-based binary logging is supported with the READ COMMITTED isolation level. If you use READ COMMITTED with binlog_format=MIXED, the server automatically uses row-based logging.

  Using READ COMMITTED has additional effects:

  • For UPDATE or DELETE statements, InnoDB holds locks only for rows that it updates or deletes. Record locks for nonmatching rows are released after MySQL has evaluated the WHERE condition. This greatly reduces the probability of deadlocks, but they can still happen.

  • For UPDATE statements, if a row is already locked, InnoDB performs a “semi-consistent” read, returning the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE. If the row matches (must be updated), MySQL reads the row again and this time InnoDB either locks it or waits for a lock on it.

  Consider the following example, beginning with this table:

  CREATE TABLE t (a INT NOT NULL, b INT) ENGINE = InnoDB;
  INSERT INTO t VALUES (1,2),(2,3),(3,2),(4,3),(5,2);
  COMMIT;

  In this case, the table has no indexes, so searches and index scans use the hidden clustered index for record locking (see Section 14.6.2.1, “Clustered and Secondary Indexes”) rather than indexed columns.

  Suppose that one session performs an UPDATE using these statements:

  # Session A
  START TRANSACTION;
  UPDATE t SET b = 5 WHERE b = 3;

  Suppose also that a second session performs an UPDATE by executing this statement following those of the first session:

  # Session B
  UPDATE t SET b = 4 WHERE b = 2;

  As InnoDB executes each UPDATE, it first acquires an exclusive lock for each row that it reads, and then determines whether to modify it. If InnoDB does not modify the row, it releases the lock. Otherwise, InnoDB retains the lock until the end of the transaction. This affects transaction processing as follows.

  When using the default REPEATABLE READ isolation level, the first UPDATE acquires an x-lock on each row that it reads and does not release any of them:

  x-lock(1,2); retain x-lock
  x-lock(2,3); update(2,3) to (2,5); retain x-lock
  x-lock(3,2); retain x-lock
  x-lock(4,3); update(4,3) to (4,5); retain x-lock
  x-lock(5,2); retain x-lock

  The second UPDATE blocks as soon as it tries to acquire any locks (because first update has retained locks on all rows), and does not proceed until the first UPDATE commits or rolls back:

  x-lock(1,2); block and wait for first UPDATE to commit or roll back

  If READ COMMITTED is used instead, the first UPDATE acquires an x-lock on each row that it reads and releases those for rows that it does not modify:

  x-lock(1,2); unlock(1,2)
  x-lock(2,3); update(2,3) to (2,5); retain x-lock
  x-lock(3,2); unlock(3,2)
  x-lock(4,3); update(4,3) to (4,5); retain x-lock
  x-lock(5,2); unlock(5,2)

  For the second UPDATE, InnoDB does a “semi-consistent” read, returning the latest committed version of each row that it reads to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE:

  x-lock(1,2); update(1,2) to (1,4); retain x-lock
  x-lock(2,3); unlock(2,3)
  x-lock(3,2); update(3,2) to (3,4); retain x-lock
  x-lock(4,3); unlock(4,3)
  x-lock(5,2); update(5,2) to (5,4); retain x-lock

  However, if the WHERE condition includes an indexed column, and InnoDB uses the index, only the indexed column is considered when taking and retaining record locks. In the following example, the first UPDATE takes and retains an x-lock on each row where b = 2. The second UPDATE blocks when it tries to acquire x-locks on the same records, as it also uses the index defined on column b.

  CREATE TABLE t (a INT NOT NULL, b INT, c INT, INDEX (b)) ENGINE = InnoDB;
  INSERT INTO t VALUES (1,2,3),(2,2,4);
  COMMIT;
  
  # Session A
  START TRANSACTION;
  UPDATE t SET b = 3 WHERE b = 2 AND c = 3;
  
  # Session B
  UPDATE t SET b = 4 WHERE b = 2 AND c = 4;

  The effects of using the READ COMMITTED isolation level are the same as enabling the deprecated innodb_locks_unsafe_for_binlog variable, with these exceptions:

  • Enabling innodb_locks_unsafe_for_binlog is a global setting and affects all sessions, whereas the isolation level can be set globally for all sessions, or individually per session.

  • innodb_locks_unsafe_for_binlog can be set only at server startup, whereas the isolation level can be set at startup or changed at runtime.

  READ COMMITTED therefore offers finer and more flexible control than innodb_locks_unsafe_for_binlog.

• READ UNCOMMITTED

  SELECT statements are performed in a nonlocking fashion, but a possible earlier version of a row might be used. Thus, using this isolation level, such reads are not consistent. This is also called a dirty read. Otherwise, this isolation level works like READ COMMITTED.

• SERIALIZABLE

  This level is like REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to SELECT ... LOCK IN SHARE MODE if autocommit is disabled. If autocommit is enabled, the SELECT is its own transaction. It therefore is known to be read only and can be serialized if performed as a consistent (nonlocking) read and need not block for other transactions. (To force a plain SELECT to block if other transactions have modified the selected rows, disable autocommit.)