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CREATE RULE

CREATE RULE — define a new rewrite rule

Synopsis

CREATE [ OR REPLACE ] RULEname AS ONevent    TOtable_name [ WHEREcondition ]    DO [ ALSO | INSTEAD ] { NOTHING |command | (command ;command ... ) }whereevent can be one of:    SELECT | INSERT | UPDATE | DELETE

Description

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.

ThePostgreSQL rule system allows one to define an alternative 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, anINSTEAD rule can replace a given command by another, or cause a command not to be executed at all. Rules are used to implement SQL 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 command 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 inChapter 40.

Presently,ON SELECT rules must be unconditionalINSTEAD rules and must have actions that consist of a singleSELECT command. Thus, anON SELECT rule effectively turns the table into a view, whose visible contents are the rows returned by the rule'sSELECT command rather than whatever had been stored in the table (if anything). It is considered better style to write aCREATE VIEW command than to create a real table and define anON SELECT rule for it.

You can create the illusion of an updatable view by definingON INSERT,ON UPDATE, andON 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. If you want to supportINSERT RETURNING and so on, then be sure to put a suitableRETURNING clause into each of these rules.

There is a catch if you try to use conditional rules for complex view updates: theremust be an unconditionalINSTEAD rule for each action you wish to allow on the view. If the rule is conditional, or is notINSTEAD, 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 unconditionalDO 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 defaultINSTEAD NOTHING action. (This method does not currently work to supportRETURNING queries, however.)

Parameters

Withincondition andcommand, the special table namesNEW andOLD can be used to refer to values in the referenced table.NEW is valid inON INSERT andON UPDATE rules to refer to the new row being inserted or updated.OLD is valid inON UPDATE andON DELETE rules to refer to the existing row being updated or deleted.

Notes

You must be the owner of a table to create or change rules for it.

In a rule forINSERT,UPDATE, orDELETE on a view, you can add aRETURNING clause that emits the view's columns. This clause will be used to compute the outputs if the rule is triggered by anINSERT RETURNING,UPDATE RETURNING, orDELETE RETURNING command respectively. When the rule is triggered by a command withoutRETURNING, the rule'sRETURNING clause will be ignored. The current implementation allows only unconditionalINSTEAD rules to containRETURNING; furthermore there can be at most oneRETURNING clause among all the rules for the same event. (This ensures that there is only one candidateRETURNING clause to be used to compute the results.)RETURNING queries on the view will be rejected if there is noRETURNING clause in any available rule.

It is very important to take care to avoid circular rules. For example, though each of the following two rule definitions are accepted byPostgreSQL, theSELECT command would causePostgreSQL to report an error because of recursive expansion of a rule:

CREATE RULE "_RETURN" AS    ON SELECT TO t1    DO INSTEAD        SELECT * FROM t2;CREATE RULE "_RETURN" AS    ON SELECT TO t2    DO INSTEAD        SELECT * FROM t1;SELECT * FROM t1;

Presently, if a rule action contains aNOTIFY command, theNOTIFY command will be executed unconditionally, that is, theNOTIFY 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;

oneNOTIFY event will be sent during theUPDATE, whether or not there are any rows that match the conditionid = 42. This is an implementation restriction that might be fixed in future releases.

Compatibility

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

See Also