GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
GNU/Linux |
RedHat 5.2(Apollo) |
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create_rule(l) |
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create rule - define a new rule
create
rule rule_name
as on event
to object [where clause]
do [instead]
[action | nothing | [actions...]]
Create rule is used to define a new rule.
Here,
event is one of select, update,
delete or insert. Object is either:
a class name
or
class.column
The from clause, the where clause, and the
action are respectively normal SQL from
clauses, where clauses and collections of SQL
commands with the following change:
new or current can appear instead of an instance variable whenever an instance variable is permissible in SQL.
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, then 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
new.attribute-name
The action part of the rule executes with same
command and transaction identifier as the user command that
caused activation.
A note of
caution about SQL rules is in order. If the same class name
or instance variable appears in the event, where
clause 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"
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do update EMP ( ... ) where ... |
on update to EMP-1.salary where EMP-2.name = "Joe"
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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 of a circular rewrite rule combination.
--
create rule bad_rule_combination_1 is
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on select to EMP | |
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do instead select to TOYEMP |
create rule bad_rule_combination_2 is
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on select to TOYEMP | |
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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 (see change acl(l).
--
--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)
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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
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select (EMP.salary) from EMP where EMP.name = "Joe" |
--
--Deny Joe access to the salary of employees in the shoe
--department. (pg_username() returns the name of the current
user)
--
create rule example_3 is
on select to EMP.salary
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where current.dept = "shoe" |
and pg_username() =
"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
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where EMP.dept = "toy" |
--
--All new employees must make 5,000 or less
--
create rule example_5 is
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on insert to EMP where new.salary > 5000 | |
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do update newset salary = 5000 |
drop rule(l), create view(l).
instead rules do not work properly.
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 store 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).
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create_rule(l) | |