CREATE RULE
SQL - Language Statements
CREATE RULE
define a new rewrite rule
2001-01-05
CREATE RULE name AS ON event
TO object [ WHERE condition ]
DO [ INSTEAD ] action
where action can be:
NOTHING
|
query
|
( query ; query ... )
|
[ query ; query ... ]
2001-01-05
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 SELECT, ON UPDATE, and ON DELETE
rules to refer to the existing row being selected, updated, or deleted.
1998-09-11
Outputs
CREATE
Message returned if the rule is successfully created.
1998-09-11
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 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.
2001-01-05
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
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 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 example, 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;
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
1998-09-11
SQL92
CREATE RULE statement is a Postgres
language extension.
There is no CREATE RULE statement in SQL92.