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.