Provided by: postgresql-client-8.0_8.0.7-2build1_i386 bug


       CREATE RULE - define a new rewrite rule


       CREATE [ OR REPLACE ] RULE name AS ON event
           TO table [ WHERE condition ]
           DO [ ALSO | INSTEAD ] { NOTHING | command | ( command ; command ... ) }


       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
       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 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.

       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.


       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

       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.

              Any   SQL   conditional   expression  (returning  boolean).  The
              condition expression may not refer to any tables except NEW  and
              OLD, and may not contain aggregate functions.

              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.

              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  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.


       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
       PostgreSQL,  the  SELECT  command  would  cause PostgreSQL to report an
       error because the query cycled too many times:

           ON SELECT TO t1
           DO INSTEAD
               SELECT * FROM t2;

           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 may be fixed in future releases.


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