SET CONSTRAINTS sets the behavior of constraint checking within the current transaction.
IMMEDIATE constraints are checked at the end of each statement.
DEFERRED constraints are not checked until transaction commit. Each constraint has its own
IMMEDIATE or
DEFERRED mode.
Upon creation, a constraint is given one of three characteristics: DEFERRABLE INITIALLY DEFERRED,
DEFERRABLE INITIALLY IMMEDIATE, or
NOT DEFERRABLE. The third class is always
IMMEDIATE and is not affected by the
SET CONSTRAINTS command. The first two classes start every transaction in the indicated mode, but their behavior can be changed within a transaction by
SET CONSTRAINTS.
SET CONSTRAINTS with a list of constraint names changes the mode of just those constraints (which must all be deferrable). If there are multiple constraints matching any given name, all are affected.
SET CONSTRAINTS ALL changes the mode of all deferrable constraints.
When SET CONSTRAINTS changes the mode of a constraint from
DEFERRED to
IMMEDIATE, the new mode takes effect retroactively: any outstanding data modifications that would have been checked at the end of the transaction are instead checked during the execution of the
SET CONSTRAINTS command. If any such constraint is violated, the
SET CONSTRAINTS fails (and does not change the constraint mode). Thus,
SET CONSTRAINTS can be used to force checking of constraints to occur at a specific point in a transaction.