CREATE [ OR REPLACE ] RULE name AS ON event
TO table [ WHERE condition ]
DO [ INSTEAD ] action
where action can be:
NOTHING
| query
| ( query ; query ... )
The name of a rule to create. This must be distinct from the name of any other rule for the same table.
Event is one of SELECT, UPDATE, DELETE or INSERT.
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.
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. 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.
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 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 table are examined successively (in order by name). 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 parentheses. Such queries will be performed in the specified order. 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.
It is important to realize that a rule is really a query transformation mechanism, or query macro. The entire query is processed to convert it into a series of queries that include the rule actions. This occurs before evaluation of the query starts. So, conditional rules are handled by adding the rule condition to the WHERE clause of the action(s) derived from the rule. The above description of a rule as an operation that executes for each row is thus somewhat misleading. If you actually want an operation that fires independently for each physical row, you probably want to use a trigger not a rule. Rules are most useful for situations that call for transforming entire queries independently of the specific data being handled.
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 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 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.
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 "_RETURN" AS
ON SELECT TO emp
DO INSTEAD
SELECT * FROM toyemp;
CREATE RULE "_RETURN" 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.