Index for Section RULE

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CREATE
NAME
  CREATE RULE - define a new rewrite rule

SYNOPSIS
  CREATE RULE name AS ON event
      TO object [ WHERE condition ]
      DO [ INSTEAD ] action

  where action can be:

  NOTHING
  |
  query
  |
  ( query ; query ... )
  |
  [ query ; query ... ]

  INPUTS

  name The name of a rule to create.

  event
       Event is one of SELECT, UPDATE, DELETE or INSERT.

  object
       Object is either table or table.column. (Currently, only the table
       form is actually implemented.)

  condition
       Any SQL boolean-condition expression. The condition expression may not
       refer to any tables except new and old.

  query
       The query or queries making up the action can be any SQL SELECT,
       INSERT, UPDATE, DELETE, or NOTIFY statement.

  Within the condition and action, the special table names new and old may be
  used to refer to values in the referenced table (the object).	 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.

  OUTPUTS

  CREATE
       Message returned if the rule is successfully created.

DESCRIPTION
  The PostgreSQL rule system allows one to define an alternate action to be
  performed on inserts, updates, or deletions from database tables. Rules are
  used to implement table views as well.

  The semantics of a rule is that at the time an individual instance (row) is
  accessed, inserted, updated, or deleted, there is an old instance (for
  selects, updates and deletes) and a new instance (for inserts and updates).
  All the rules for the given event type and the given target object (table)
  are examined, in an unspecified order. If the condition specified in the
  WHERE clause (if any) is true, the action part of the rule is executed. The
  action is done instead of the original query if INSTEAD is specified;
  otherwise it is done after the original query in the case of ON INSERT, or
  before the original query in the case of ON UPDATE or ON DELETE.  Within
  both the condition and action, values from fields in the old instance
  and/or the new instance are substituted for old.attribute-name and
  new.attribute-name.

  The action part of the rule can consist of one or more queries. To write
  multiple queries, surround them with either parentheses or square brackets.
  Such queries will be performed in the specified order (whereas there are no
  guarantees about the execution order of multiple rules for an object). The
  action can also be NOTHING indicating no action. Thus, a DO INSTEAD NOTHING
  rule suppresses the original query from executing (when its condition is
  true); a DO NOTHING rule is useless.

  The action part of the rule executes with the same command and transaction
  identifier as the user command that caused activation.

  RULES AND VIEWS

  Presently, ON SELECT rules must be unconditional INSTEAD rules and must
  have actions that consist of a single SELECT query. Thus, an ON SELECT rule
  effectively turns the object table into a view, whose visible contents are
  the rows returned by the rule's SELECT query 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.

  create_view(5) creates a dummy table (with no underlying storage) and
  associates an ON SELECT rule with it. The system will not allow updates to
  the view, since it knows there is no real table there.  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 in
  some cases.  If you want to handle all the useful cases in conditional
  rules, you can; just 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
  fire, they add to the default INSTEAD NOTHING action.

  NOTES

  You must have rule definition access to a table in order to define a rule
  on it. Use GRANT and REVOKE to change permissions.

  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 will cause PostgreSQL to report an error
  because the query cycled too many times:

  CREATE RULE "_RETemp" AS
      ON SELECT TO emp
      DO INSTEAD
	  SELECT * FROM toyemp;

  CREATE RULE "_RETtoyemp" AS
      ON SELECT TO toyemp
      DO INSTEAD
	  SELECT * FROM emp;

  This attempt to select from EMP will cause PostgreSQL to issue an error
  because the queries cycled too many times:

  SELECT * FROM emp;

  Presently, if a rule contains a NOTIFY query, the NOTIFY 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 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 with id = 42. This is an implementation restriction that may be
  fixed in future releases.

COMPATIBILITY
  SQL92

  CREATE RULE statement is a PostgreSQL language extension.  There is no
  CREATE RULE statement in SQL92.

Index for Section RULE

Alphabetical listing for C

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