SET CONSTRAINTS
SET CONSTRAINTS — set constraint check timing for the current transaction
Synopsis
SET CONSTRAINTS { ALL | name [, ...] } { DEFERRED | IMMEDIATE }
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). Each constraint name can be schema-
qualified. The current schema search path is used to find the first matching name
if no schema name is specified. 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
he constraint mode). Thus, SET CONSTRAINTS can be used to force checking of
constraints to occur at a specific point in a transaction.
Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and EXCLUDE constraints are affected by this setting. NOT NULL and CHECK constraints are always checked immediately when a row is inserted or modified (not at the end of the statement). Uniqueness and exclusion constraints that have not been declared DEFERRABLE are also checked immediately.
The firing of triggers that are declared as “constraint triggers” is also controlled by this setting — they fire at the same time that the associated constraint should be checked.
Notes
Because QHB does not require constraint names to be unique
within a schema (but only per-table), it is possible that there is more than one
match for a specified constraint name. In this case SET CONSTRAINTS will act
on all matches. For a non-schema-qualified name, once a match or matches have
been found in some schema in the search path, schemas appearing later in the path
are not searched.
This command only alters the behavior of constraints within the current transaction. Issuing this outside of a transaction block emits a warning and otherwise has no effect.
Compatibility
This command complies with the behavior defined in the SQL standard, except for the limitation that, in QHB, it does not apply to NOT NULL and CHECK constraints. Also, QHB checks non-deferrable uniqueness constraints immediately, not at end of statement as the standard would suggest.