SET CONSTRAINTS sets the behavior of
constraint evaluation in the current transaction. In IMMEDIATE mode, constraints are checked at the
end of each statement. In DEFERRED
mode, constraints are not checked until transaction commit.
Note: This command only alters the behavior of constraints within the current transaction. Thus, if you execute this command outside of an explicit transaction block (such as one started with BEGIN), it will not appear to have any effect. If you wish to change the behavior of a constraint without needing to issue a SET CONSTRAINTS command in every transaction, specify
INITIALLY DEFERREDorINITIALLY IMMEDIATEwhen you create the constraint.
When you change the mode of a constraint to be IMMEDIATE , the new constraint mode takes effect
retroactively: any outstanding data modifications that would have
been checked at the end of the transaction (when using
DEFERRED) are instead checked during
the execution of the SET CONSTRAINTS
command.
Upon creation, a constraint is always give one of three
characteristics: INITIALLY DEFERRED,
INITIALLY IMMEDIATE DEFERRABLE, or
INITIALLY IMMEDIATE NOT DEFERRABLE.
The third class is not affected by the SET
CONSTRAINTS command.
Currently, only foreign key constraints are affected by this setting. Check and unique constraints are always effectively initially immediate not deferrable.