CREATE 
RULE(7)							   SQL Commands 						    CREATE RULE(7)

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. Alterna-
       tively, 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 transforma-
       tion 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 ta-
       ble 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  suf-
       ficient	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 can-
	      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 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 implementa-
       tion restriction that might be fixed in future releases.

COMPATIBILITY

       CREATE RULE is a PostgreSQL language extension, as is the entire query rewrite system.

SQL - Language Statements					    2010-05-14							    CREATE RULE(7)