CREATE RULE(7) PostgreSQL 15.7 Documentation CREATE RULE(7)
NAME
CREATE_RULE - define a new rewrite rule
SYNOPSIS
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 Chapter 41.
Presently, ON SELECT rules can only be attached to views. (Attaching
one to a table converts the table into a view.) Such a rule must be
named "_RETURN", must be an unconditional INSTEAD rule, and must have
an action that consists of a single SELECT command. This command
defines the visible contents of the view. (The view itself is basically
a dummy table with no storage.) It's best to regard such a rule as an
implementation detail. While a view can be redefined via CREATE OR
REPLACE RULE "_RETURN" AS ..., it's better style to use CREATE OR
REPLACE VIEW.
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
CREATE VIEW (CREATE_VIEW(7))) 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 CREATE TRIGGER (CREATE_TRIGGER(7))) 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
ALTER RULE (ALTER_RULE(7)), DROP RULE (DROP_RULE(7))
PostgreSQL 15.7 2024 CREATE RULE(7)
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