NAME
CREATE RULE - define a new rewrite rule
SYNOPSIS
CREATE [ OR REPLACE ] RULE name AS ON event
TO table [ WHERE condition ]
DO [ ALSO | INSTEAD ] { NOTHING | command | ( command ; command ... ) }
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 table 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 commands 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 in the documentation.
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 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.)
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.
table 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.