SET CONSTRAINTS

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 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 statement. The first two classes start every transaction in specified modes, but its behaviors 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 multiple constraints match a name, the name is affected by all of these constraints. SET CONSTRAINTS ALL changes the modes of all deferrable constraints.

When SET CONSTRAINTS changes the mode of a constraint from DEFERRED to IMMEDIATE, any data that is modified at the end of the transaction is checked during the execution of the SET CONSTRAINTS statement. If any such constraint is violated, the SET CONSTRAINTS fails (and does not change the constraint mode). Therefore, SET CONSTRAINTS can be used to force checking of constraints to occur at a specific point in a transaction.

Precautions

SET CONSTRAINTS sets the behavior of constraint checking only within the current transaction. Therefore, if you execute this statement outside of a transaction block (START TRANSACTION/COMMIT pair), it will not appear to have any effect.

Syntax

SET CONSTRAINTS { ALL | { name } [, ...] } { DEFERRED | IMMEDIATE };

Parameters

• name

  Specifies the constraint name.

  Value range: an existing table name, which can be found in the system catalog pg_constraint.

• ALL

  Specifies all constraints.

• DEFERRED

  Specifies that constraints are not checked until transaction commit.

• IMMEDIATE

  Specifies that constraints are checked at the end of each statement.

Examples

-- Set that constraints are checked when a transaction is committed.
gaussdb=# SET CONSTRAINTS ALL DEFERRED;