Description
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.
Currently, only foreign key constraints are affected by this
setting. Check and unique constraints are always effectively
not deferrable.
Notes
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.
Compatibility
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.
The SQL standard says that constraint names appearing in SET
CONSTRAINTS can be schema-qualified. This is not yet
supported by PostgreSQL: the names must
be unqualified, and all constraints matching the command will be
affected no matter which schema they are in.
