CREATE RULE

Defines a new rewrite rule.

Synopsis

CREATE [OR REPLACE] RULE <name> AS ON <event>
  TO <table> [WHERE <condition>] 
  DO [ALSO | INSTEAD] { NOTHING | <command> | (<command>; <command> 
  ...) }

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.

The Greenplum Database rule system allows one to define an alternate action to be performed on insertions, updates, or deletions in database tables. A rule causes additional or alternate commands to be executed when a given command on a given table is executed. Rules can be used on 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 commands starts. It does not operate independently for each physical row as does a trigger.

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

You can create the illusion of an updatable view by defining ON INSERT, ON UPDATE, and ON DELETE rules 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 of the view in some cases. If you want to handle all the useful cases in conditional rules, 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 are applied, they add to the default INSTEAD NOTHING action. (This method does not currently work to support RETURNING queries, however.)

Parameters

name: The name of a rule to create. This must be distinct from the name of any other rule for the same table. Multiple rules on the same table and same event type are applied in alphabetical name order.
event: The event is one of SELECT, INSERT, UPDATE, or DELETE.
table: The name (optionally schema-qualified) of the table or view the rule applies to.
condition: 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. NEW and OLD 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.
INSTEAD: INSTEAD indicates that the commands should be executed instead of the original command.
ALSO: ALSO indicates that the commands should be executed in addition to the original command. If neither ALSO nor INSTEAD is specified, ALSO is the default.
command: The command or commands that make up the rule action. Valid commands are SELECT, INSERT, UPDATE, or DELETE. 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.

Notes

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

It is very important to take care to avoid circular rules. Recursive rules are not validated at rule create time, but will report an error at execution time.

Examples

Create a rule that inserts rows into the child table b2001 when a user tries to insert into the partitioned parent table rank:

CREATE RULE b2001 AS ON INSERT TO rank WHERE gender='M' and 
year='2001' DO INSTEAD INSERT INTO b2001 VALUES (NEW.id, 
NEW.rank, NEW.year, NEW.gender, NEW.count);

Compatibility

CREATE RULE is a Greenplum Database language extension, as is the entire query rewrite system.