CREATE RULE() SQL Commands CREATE RULE()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 alternate 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 exe‐
cution 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 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 con‐
tents 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 suffi‐
cient for your purposes) to replace update actions on the view with
appropriate updates on other tables.
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.
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 condi‐
tion expression may not refer to any tables except NEW and OLD,
and may not 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 com‐
mands are SELECT, INSERT, UPDATE, DELETE, or NOTIFY.
Within condition and command, the special table names NEW and OLD may
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 have the privilege RULE on a table to be allowed to define a
rule on it.
It is very important to take care to avoid circular rules. For example,
though each of the following two rule definitions are accepted by Post‐
greSQL, the SELECT command would cause PostgreSQL to report an error
because the query cycled too many times:
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 com‐
mand 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 implementa‐
tion restriction that may be fixed in future releases.
COMPATIBILITY
CREATE RULE is a PostgreSQL language extension, as is the entire query
rewrite system.
SQL - Language Statements 2008-01-03 CREATE RULE()
