SET CONSTRAINTS

SET CONSTRAINTS

7

SQL - Language Statements

SET CONSTRAINTS

set constraint check timing for the current transaction

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 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 `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 MDASH they fire at the same time that
the associated constraint should be checked.

Notes
=====

Because PostgreSQL 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 PostgreSQL, it does not apply to
`NOT NULL` and `CHECK` constraints. Also, PostgreSQL checks
non-deferrable uniqueness constraints immediately, not at end of
statement as the standard would suggest.
