Description
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 DEFERRED or
INITIALLY IMMEDIATE when 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.
