CREATE RULE

CREATE RULE

7

SQL - Language Statements

CREATE RULE

define a new rewrite rule

CREATE \[ OR REPLACE \] RULE

name

AS ON

event

TO

table\_name

\[ WHERE

condition

\] DO \[ ALSO \| INSTEAD \] { NOTHING \|

command

\| (

command

;

command

\... ) }

where

event

can be one of:

SELECT \| INSERT \| UPDATE \| DELETE

Description
===========

`CREATE RULE` defines a new rule applying to a specified table or view.
`CREATE OR REPLACE RULE` will either create a new rule, or replace an
existing rule of the same name for the same table.

The PostgreSQL rule system allows one to define an alternative action to
be performed on insertions, updates, or deletions in database tables.
Roughly speaking, a rule causes additional commands to be executed when
a given command on a given table is executed. Alternatively, an
`INSTEAD` rule can replace a given command by another, or cause a
command not to be executed at all. Rules are used to implement SQL views
as well. It is important to realize that a rule is really a command
transformation mechanism, or command macro. The transformation happens
before the execution of the command starts. If you actually want an
operation that fires independently for each physical row, you probably
want to use a trigger, not a rule. More information about the rules
system is in [???](#rules).

Presently, `ON SELECT` rules must be unconditional `INSTEAD` rules and
must have actions that consist of a single `SELECT` command. Thus, an
`ON SELECT` rule effectively turns the table into a view, whose visible
contents are the rows returned by the rule\'s `SELECT` command rather
than whatever had been stored in the table (if anything). It is
considered better style to write a `CREATE VIEW` command than to create
a real table and define an `ON SELECT` rule for it.

You can create the illusion of an updatable view by defining
`ON INSERT`, `ON UPDATE`, and `ON DELETE` rules (or any subset of those
that\'s sufficient for your purposes) to replace update actions on the
view with appropriate updates on other tables. If you want to support
`INSERT RETURNING` and so on, then be sure to put a suitable `RETURNING`
clause into each of these rules.

There is a catch if you try to use conditional rules for complex view
updates: there *must* be an unconditional `INSTEAD` rule for each action
you wish to allow on the view. If the rule is conditional, or is not
`INSTEAD`, then the system will still reject attempts to perform the
update action, because it thinks it might end up trying to perform the
action on the dummy table of the view in some cases. If you want to
handle all the useful cases in conditional rules, add an unconditional
`DO
   INSTEAD NOTHING` rule to ensure that the system understands it will
never be called on to update the dummy table. Then make the conditional
rules non-`INSTEAD`; in the cases where they are applied, they add to
the default `INSTEAD NOTHING` action. (This method does not currently
work to support `RETURNING` queries, however.)

> **Note**
>
> A view that is simple enough to be automatically updatable (see
> [???](#sql-createview)) does not require a user-created rule in order
> to be updatable. While you can create an explicit rule anyway, the
> automatic update transformation will generally outperform an explicit
> rule.
>
> Another alternative worth considering is to use `INSTEAD OF` triggers
> (see [???](#sql-createtrigger)) in place of rules.

Parameters
==========

name

:   The name of a rule to create. This must be distinct from the name of
    any other rule for the same table. Multiple rules on the same table
    and same event type are applied in alphabetical name order.

event

:   The event is one of `SELECT`, `INSERT`, `UPDATE`, or `DELETE`. Note
    that an `INSERT` containing an `ON
          CONFLICT` clause cannot be used on tables that have either
    `INSERT` or `UPDATE` rules. Consider using an updatable view
    instead.

table\_name

:   The name (optionally schema-qualified) of the table or view the rule
    applies to.

condition

:   Any SQL conditional expression (returning `boolean`). The condition
    expression cannot refer to any tables except `NEW` and `OLD`, and
    cannot contain aggregate functions.

`INSTEAD`

:   `INSTEAD` indicates that the commands should be executed *instead
    of* the original command.

`ALSO`

:   `ALSO` indicates that the commands should be executed *in addition
    to* the original command.

    If neither `ALSO` nor `INSTEAD` is specified, `ALSO` is the default.

command

:   The command or commands that make up the rule action. Valid commands
    are `SELECT`, `INSERT`, `UPDATE`, `DELETE`, or `NOTIFY`.

Within condition and command, the special table names `NEW` and `OLD`
can be used to refer to values in the referenced table. `NEW` is valid
in `ON INSERT` and `ON UPDATE` rules to refer to the new row being
inserted or updated. `OLD` is valid in `ON UPDATE` and `ON DELETE` rules
to refer to the existing row being updated or deleted.

Notes
=====

You must be the owner of a table to create or change rules for it.

In a rule for `INSERT`, `UPDATE`, or `DELETE` on a view, you can add a
`RETURNING` clause that emits the view\'s columns. This clause will be
used to compute the outputs if the rule is triggered by an
`INSERT RETURNING`, `UPDATE RETURNING`, or `DELETE RETURNING` command
respectively. When the rule is triggered by a command without
`RETURNING`, the rule\'s `RETURNING` clause will be ignored. The current
implementation allows only unconditional `INSTEAD` rules to contain
`RETURNING`; furthermore there can be at most one `RETURNING` clause
among all the rules for the same event. (This ensures that there is only
one candidate `RETURNING` clause to be used to compute the results.)
`RETURNING` queries on the view will be rejected if there is no
`RETURNING` clause in any available rule.

It is very important to take care to avoid circular rules. For example,
though each of the following two rule definitions are accepted by
PostgreSQL, the `SELECT` command would cause PostgreSQL to report an
error because of recursive expansion of a rule:

    CREATE RULE "_RETURN" AS
        ON SELECT TO t1
        DO INSTEAD
            SELECT * FROM t2;

    CREATE RULE "_RETURN" AS
        ON SELECT TO t2
        DO INSTEAD
            SELECT * FROM t1;

    SELECT * FROM t1;

Presently, if a rule action contains a `NOTIFY` command, the `NOTIFY`
command will be executed unconditionally, that is, the `NOTIFY` will be
issued even if there are not any rows that the rule should apply to. For
example, in:

    CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;

    UPDATE mytable SET name = 'foo' WHERE id = 42;

one `NOTIFY` event will be sent during the `UPDATE`, whether or not
there are any rows that match the condition `id = 42`. This is an
implementation restriction that might be fixed in future releases.

Compatibility
=============

`CREATE RULE` is a PostgreSQL language extension, as is the entire query
rewrite system.

See Also
========
