PostgreSQL 8.0.0rc4 Documentation
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# SET CONSTRAINTS

## Name

SET CONSTRAINTS -- set constraint checking modes 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). 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.

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