CREATE 
RULE(7)						  PostgreSQL 9.2.7 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 ... ) }

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 9.2.7						    2014-02-17							    CREATE RULE(7)