innodb_locks_unsafe_for_binlog

disable next-key lock

This variable affects how

InnoDB

uses gap locking for searches and index scans. Normally,

InnoDB

uses an algorithm called next-key locking that combines index-row locking with gap locking.

InnoDB

performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record

R

in an index, another session cannot insert a new index record in the gap immediately before

R

in the index order.

By default, the value of

innodb_locks_unsafe_for_binlog

is 0 (disabled), which means that gap locking is enabled:

InnoDB

uses next-key locks for searches and index scans. To enable the variable, set it to 1. This causes gap locking to be disabled:

InnoDB

uses only index-record locks for searches and index scans.

The effect of enabling

innodb_locks_unsafe_for_binlog

is similar to but not identical to setting the transaction isolation level to

READ           COMMITTED

:

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.