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.
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.
This command only alters the behavior of constraints within the current transaction. Thus, if you execute this command outside of a transaction block (BEGIN/COMMIT pair), 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.
This command complies with the behavior defined in the SQL standard, except for the limitation that, in PostgreSQL, it only applies to foreign-key constraints.