CREATE RULE() SQL Commands CREATE RULE()NAME
CREATE RULE - Defines a new 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 ta‐
ble 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 SELECT, ON UPDATE, and
ON DELETE rules to refer to the existing row being selected, updated,
or deleted.
OUTPUTS
CREATE Message returned if the rule is successfully created.
DESCRIPTION
The Postgres 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 before the
original query is performed. Within both the condition and action,
values from fields in the old instance and/or the new instance are sub‐
stituted 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 use‐
less.
The action part of the rule executes with the same command and transac‐
tion identifier as the user command that caused activation.
NOTES
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 con‐
tents 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 table and
define an ON SELECT rule for it.
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 exam‐
ple, though each of the following two rule definitions are accepted by
Postgres, the select command will cause Postgres to report an error
because the query cycled too many times:
EXAMPLE OF A CIRCULAR REWRITE RULE COMBINATION:
CREATE RULE bad_rule_combination_1 AS
ON SELECT TO emp
DO INSTEAD
SELECT * FROM toyemp;
CREATE RULE bad_rule_combination_2 AS
ON SELECT TO toyemp
DO INSTEAD
SELECT * FROM emp;
This attempt to select from EMP will cause Postgres to issue an error
because the queries cycled too many times:
SELECT * FROM emp;
COMPATIBILITY
SQL92
CREATE RULE statement is a Postgres language extension. There is no
CREATE RULE statement in SQL92.
SQL - Language Statements 29 March 2001 CREATE RULE()