CREATE RULE
PostgreSQL User's GuidePrevNextCREATE RULEName CREATE RULE
— Defines a new rule
CREATE RULE name
AS ON event
TO object [ WHERE condition ]
DO [ INSTEAD ] [ action | NOTHING ]
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.
condition Any SQL WHERE clause. new or
current can appear instead of an instance
variable whenever an instance variable is permissible in SQL.
action Any SQL statement. new or
current can appear instead of an instance
variable whenever an instance variable is permissible in SQL.
Outputs
CREATE Message returned if the rule is successfully created.
Description
The semantics of a rule is that at the time an individual instance is
accessed, updated, inserted or deleted, there is a current instance (for
retrieves, updates and deletes) and a new instance (for updates and
appends). If the event
specified in the ON clause and the
condition specified in the
WHERE clause are true for the current instance, the
action part of the rule is
executed. First, however, values from fields in the current instance
and/or the new instance are substituted for
current.attribute-name
and new.attribute-name.
The action part of the rule
executes with the same command and transaction identifier as the user
command that caused activation.
Notes
A caution about SQL rules is in order. If the same class name
or instance variable appears in the
event, the
condition and the
action parts of a rule,
they are all considered different tuple variables. More accurately,
new and current are the only tuple
variables that are shared between these clauses. For example, the following
two rules have the same semantics:
on update to EMP.salary where EMP.name = "Joe"
do update EMP ( ... ) where ...
on update to EMP-1.salary where EMP-2.name = "Joe"
do update EMP-3 ( ... ) where ...
Each rule can have the optional tag INSTEAD.
Without
this tag, action will be
performed in addition to the user command when the
event in the
condition part of the rule
occurs. Alternately, the
action part will be done
instead of the user command. In this later case, the
action can be the keyword
NOTHING.
When choosing between the rewrite and instance rule systems for a
particular rule application, remember that in the rewrite system,
current refers to a relation and some qualifiers
whereas in the instance system it refers to an instance (tuple).
It is very important to note that the rewrite rule system
will neither detect nor process circular rules. For example, though each
of the following two rule definitions are accepted by
Postgres, the
retrieve command will cause Postgres to crash:
Example 16-1. Example of a circular rewrite rule combination.create rule bad_rule_combination_1 is
on select to EMP
do instead select to TOYEMP
create rule bad_rule_combination_2 is
on select to TOYEMP
do instead select to EMP
This attempt to retrieve from EMP will cause
Postgres to crash.
select * from EMP
You must have rule definition access to a class in order
to define a rule on it. Use GRANT
and REVOKE to change permissions.
Usage
Make Sam get the same salary adjustment as Joe:
create rule example_1 is
on update EMP.salary where current.name = "Joe"
do update EMP (salary = new.salary)
where EMP.name = "Sam"
At the time Joe receives a salary adjustment, the event
will become true and Joe's current instance and proposed
new instance are available to the execution routines.
Hence, his new salary is substituted into the action part
of the rule which is subsequently executed. This propagates
Joe's salary on to Sam.
Make Bill get Joe's salary when it is accessed:
create rule example_2 is
on select to EMP.salary
where current.name = "Bill"
do instead
select (EMP.salary) from EMP
where EMP.name = "Joe"
Deny Joe access to the salary of employees in the shoe
department (current_user returns the name of
the current user):
create rule example_3 is
on select to EMP.salary
where current.dept = "shoe" and current_user = "Joe"
do instead nothing
Create a view of the employees working in the toy department.
create TOYEMP(name = char16, salary = int4)
create rule example_4 is
on select to TOYEMP
do instead
select (EMP.name, EMP.salary) from EMP
where EMP.dept = "toy"
All new employees must make 5,000 or less
create rule example_5 is
on insert to EMP where new.salary > 5000
do update newset salary = 5000
Bugs
The object in a SQL rule cannot be an array reference and
cannot have parameters.
Aside from the "oid" field, system attributes cannot be
referenced anywhere in a rule. Among other things, this
means that functions of instances (e.g., "foo(emp)" where
"emp" is a class) cannot be called anywhere in a rule.
The rule system stores the rule text and query plans as
text attributes. This implies that creation of rules may
fail if the rule plus its various internal representations
exceed some value that is on the order of one page (8KB).
Compatibility
CREATE RULE statement is a Postgres
language extension.
SQL92
There is no CREATE RULE statement in SQL92.
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