MySQL locks

For example, SELECT ... LOCK IN SHARE MODE sets an IS lock and SELECT ... FOR UPDATE sets an IX lock.

The intention locking protocol is as follows:

• Before a transaction can acquire an S lock on a row in table t , it must first acquire an IS or stronger lock on t .

• Before a transaction can acquire an X lock on a row, it must first acquire an IX lock on t .

These rules can be conveniently summarized by means of the following lock type compatibility matrix .

	X	IX	S	IS
X	Conflict	Conflict	Conflict	Conflict
IX	Conflict	Compatible	Conflict	Compatible
S	Conflict	Conflict	Compatible	Compatible
IS	Conflict	Compatible	Compatible	Compatible