Provided by: postgres-xc-client_1.1-2ubuntu2_amd64 
NAME
CREATE_RULE - define a new rewrite rule
SYNOPSIS
CREATE [ OR REPLACE ] RULE name AS ON event
TO table_name [ WHERE condition ]
DO [ ALSO | INSTEAD ] { NOTHING | command | ( command ; command ... ) }
DESCRIPTION
Note
The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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.
Postgres-XC inherits the rule system from PostgreSQL which 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, an
INSTEAD rule can replace a given command by another, or cause a command not to be executed at all. Rules
are used to implement table 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. 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 in Chapter 38, The
Rule System, in the documentation.
Presently, 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
(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 support INSERT RETURNING and so on, then be sure to
put a suitable RETURNING clause into each of these rules. Alternatively, an updatable view can be
implemented using INSTEAD OF triggers (see CREATE TRIGGER (CREATE_TRIGGER(7))).
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_name
The name (optionally schema-qualified) of the table or view the rule applies to.
condition
Any SQL conditional expression (returning boolean). The condition expression cannot refer to any
tables except NEW and OLD, and cannot contain aggregate functions.
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,
DELETE, or NOTIFY.
Within condition and command, the special table names NEW and OLD can 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.
In a rule for INSERT, UPDATE, or DELETE on a view, you can add a RETURNING clause that emits the view's
columns. This clause will be used to compute the outputs if the rule is triggered by an INSERT RETURNING,
UPDATE RETURNING, or DELETE RETURNING command respectively. When the rule is triggered by a command
without RETURNING, the rule's RETURNING clause will be ignored. The current implementation allows only
unconditional INSTEAD rules to contain RETURNING; furthermore there can be at most one RETURNING clause
among all the rules for the same event. (This ensures that there is only one candidate RETURNING clause
to be used to compute the results.) RETURNING queries on the view will be rejected if there is no
RETURNING 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 by Postgres-XC, the SELECT command would cause Postgres-XC 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 a NOTIFY command, the NOTIFY command 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 ALSO 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 that match the
condition id = 42. This is an implementation restriction that might be fixed in future releases.
COMPATIBILITY
CREATE RULE is a Postgres-XC language extension, as is the entire query rewrite system.