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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	  2010-12-14			 CREATE RULE()
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