CREATE RULE(7)		PostgreSQL 9.2.24 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 ... ) }

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 Chapter 37, The Rule System,
       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. Alternatively, an updatable view can be
       implemented using INSTEAD OF triggers (see CREATE TRIGGER
       (CREATE_TRIGGER(7))).

       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_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.



PostgreSQL 9.2.24		  2017-11-06			CREATE RULE(7)