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Bring CREATE RULE reference page into some semblance of agreement with

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what's actually implemented.
This commit is contained in:
Tom Lane 2001-01-06 04:14:35 +00:00
parent a6944611e2
commit 96bd67f61d
1 changed files with 82 additions and 155 deletions
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237
doc/src/sgml/ref/create_rule.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.20 2000/12/12 16:47:52 momjian Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.21 2001/01/06 04:14:35 tgl Exp $
Postgres documentation
-->
@ -20,17 +20,27 @@ Postgres documentation
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>1999-07-20</date>
<date>2001-01-05</date>
</refsynopsisdivinfo>
<synopsis>
CREATE RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable class="parameter">event</replaceable>
TO <replaceable class="parameter">object</replaceable> [ WHERE <replaceable class="parameter">condition</replaceable> ]
DO [ INSTEAD ] [ <replaceable class="parameter">action</replaceable> | NOTHING ]
DO [ INSTEAD ] <replaceable class="parameter">action</replaceable>
where <replaceable class="PARAMETER">action</replaceable> can be:
NOTHING
|
<replaceable class="parameter">query</replaceable>
|
( <replaceable class="parameter">query</replaceable> ; <replaceable class="parameter">query</replaceable> ... )
|
[ <replaceable class="parameter">query</replaceable> ; <replaceable class="parameter">query</replaceable> ... ]
</synopsis>
<refsect2 id="R2-SQL-CREATERULE-1">
<refsect2info>
<date>1998-09-11</date>
<date>2001-01-05</date>
</refsect2info>
<title>
Inputs
@ -50,9 +60,9 @@ CREATE RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable
<term><replaceable class="parameter">event</replaceable></term>
<listitem>
<para>
Event is one of <literal>select</literal>,
<literal>update</literal>, <literal>delete</literal>
or <literal>insert</literal>.
Event is one of <literal>SELECT</literal>,
<literal>UPDATE</literal>, <literal>DELETE</literal>
or <literal>INSERT</literal>.
</para>
</listitem>
</varlistentry>
@ -62,7 +72,9 @@ CREATE RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable
<para>
Object is either <replaceable class="parameter">table</replaceable>
or <replaceable class="parameter">table</replaceable>.<replaceable
class="parameter">column</replaceable>.
class="parameter">column</replaceable>. (Currently, only the
<replaceable class="parameter">table</replaceable> form is
actually implemented.)
</para>
</listitem>
</varlistentry>
@ -70,24 +82,38 @@ CREATE RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable
<term><replaceable class="parameter">condition</replaceable></term>
<listitem>
<para>
Any SQL WHERE clause, <literal>new</literal> or
<literal>old</literal>, can appear instead of an instance
variable whenever an instance variable is permissible in SQL.
Any SQL boolean-condition expression. The condition expression may not
refer to any tables except <literal>new</literal> and
<literal>old</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">action</replaceable></term>
<term><replaceable class="parameter">query</replaceable></term>
<listitem>
<para>
Any SQL statement, <literal>new</literal> or
<literal>old</literal>, can appear instead of an instance
variable whenever an instance variable is permissible in SQL.
The query or queries making up the
<replaceable class="PARAMETER">action</replaceable>
can be any SQL <literal>SELECT</literal>, <literal>INSERT</literal>,
<literal>UPDATE</literal>, <literal>DELETE</literal>, or
<literal>NOTIFY</literal> statement.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Within the <replaceable class="parameter">condition</replaceable>
and <replaceable class="PARAMETER">action</replaceable>, the special
table names <literal>new</literal> and <literal>old</literal> may be
used to refer to values in the referenced table (the
<replaceable class="parameter">object</replaceable>).
<literal>new</literal> is valid in ON INSERT and ON UPDATE rules
to refer to the new row being inserted or updated.
<literal>old</literal> is valid in ON SELECT, ON UPDATE, and ON DELETE
rules to refer to the existing row being selected, updated, or deleted.
</para>
</refsect2>
<refsect2 id="R2-SQL-CREATERULE-2">
@ -127,26 +153,42 @@ CREATE
The <productname>Postgres</productname>
<firstterm>rule system</firstterm> allows one to define an
alternate action to be performed on inserts, updates, or deletions
from database tables or classes. Currently, rules are used to
implement table views.
from database tables. Rules are used to
implement table views as well.
</para>
<para>
The semantics of a rule is that at the time an individual instance is
The semantics of a rule is that at the time an individual instance (row)
is
accessed, inserted, updated, or deleted, there is an old instance (for
selects, updates and deletes) and a new instance (for inserts and
updates).
If the <replaceable class="parameter">event</replaceable>
specified in the ON clause and the
updates). All the rules for the given event type and the given target
object (table) are examined, in an unspecified order. If the
<replaceable class="parameter">condition</replaceable> specified in the
WHERE clause are true for the old instance, the
WHERE clause (if any) is true, the
<replaceable class="parameter">action</replaceable> part of the rule is
executed. First, however, values from fields in the old instance
and/or the new instance are substituted for
executed. The <replaceable class="parameter">action</replaceable> is
done instead of the original query if INSTEAD is specified; otherwise
it is done before the original query is performed.
Within both the <replaceable class="parameter">condition</replaceable>
and <replaceable class="parameter">action</replaceable>, values from
fields in the old instance and/or the new instance are substituted for
<literal>old.</literal><replaceable class="parameter">attribute-name</replaceable>
and <literal>new.</literal><replaceable class="parameter">attribute-name</replaceable>.
</para>
<para>
The <replaceable class="parameter">action</replaceable> part of the rule
can consist of one or more queries. To write multiple queries, surround
them with either parentheses or square brackets. Such queries will be
performed in the specified order (whereas there are no guarantees about
the execution order of multiple rules for an object). The
<replaceable class="parameter">action</replaceable> can also be NOTHING
indicating no action. Thus, a DO INSTEAD NOTHING rule suppresses the
original query from executing (when its condition is true); a DO NOTHING
rule is useless.
</para>
<para>
The <replaceable class="parameter">action</replaceable> part of the rule
executes with the same command and transaction identifier as the user
@ -155,47 +197,29 @@ CREATE
<refsect2 id="R2-SQL-CREATERULE-3">
<refsect2info>
<date>1998-09-11</date>
<date>2001-01-05</date>
</refsect2info>
<title>
Notes
</title>
<para>
A caution about SQL rules is in order. If the same class name
or instance variable appears in the
<replaceable class="parameter">event</replaceable>,
<replaceable class="parameter">condition</replaceable> and
<replaceable class="parameter">action</replaceable> parts of a rule,
they are all considered different tuple variables. More accurately,
<literal>new</literal> and <literal>old</literal> are the only tuple
variables that are shared between these clauses. For example, the following
two rules have the same semantics:
<programlisting>
ON UPDATE TO emp.salary WHERE emp.name = "Joe"
DO
UPDATE emp SET ... WHERE ...
</programlisting>
<programlisting>
ON UPDATE TO emp-1.salary WHERE emp-2.name = "Joe"
DO
UPDATE emp-3 SET ... WHERE ...
</programlisting>
Each rule can have the optional tag INSTEAD.
Without
this tag, <replaceable class="parameter">action</replaceable> will be
performed in addition to the user command when the
<replaceable class="parameter">event</replaceable> in the
<replaceable class="parameter">condition</replaceable> part of the rule
occurs. Alternately, the
<replaceable class="parameter">action</replaceable> part will be done
instead of the user command. In this latter case, the
<replaceable class="parameter">action</replaceable> can be the keyword
<literal>NOTHING</literal>.
Presently, ON SELECT rules must be unconditional INSTEAD rules and must
have actions that consist of a single SELECT query. Thus, an ON SELECT
rule effectively turns the object table into a view, whose visible
contents are the rows returned by the rule's SELECT query 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 table and
define an ON SELECT rule for it.
</para>
<para>
It is very important to note to avoid circular rules.
You must have rule definition access to a class in order
to define a rule on it. Use <command>GRANT</command>
and <command>REVOKE</command> to change permissions.
</para>
<para>
It is very important to take care to avoid circular rules.
For example, though each
of the following two rule definitions are accepted by
<productname>Postgres</productname>, the
@ -226,105 +250,8 @@ SELECT * FROM emp;
</programlisting></para>
</example>
</para>
<para>
You must have rule definition access to a class in order
to define a rule on it. Use <command>GRANT</command>
and <command>REVOKE</command> to change permissions.
</para>
<para>
The object in a <acronym>SQL</acronym> rule cannot be an array reference and
cannot have parameters.
</para>
<para>
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., <literal>foo(emp)</literal> where
<literal>emp</literal> is a class) cannot be called anywhere in a rule.
</para>
<para>
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).
</para>
</refsect2>
</refsect1>
<refsect1 id="R1-SQL-CREATERULE-2">
<title>
Usage
</title>
<para>
Make Sam get the same salary adjustment as Joe:
<programlisting>
CREATE RULE example_1 AS
ON UPDATE emp.salary WHERE old.name = "Joe"
DO
UPDATE emp
SET salary = new.salary
WHERE emp.name = "Sam";
</programlisting>
At the time Joe receives a salary adjustment, the event
will become true and Joe's old 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.
</para>
<para>
Make Bill get Joe's salary when it is accessed:
<programlisting>
CREATE RULE example_2 AS
ON SELECT TO EMP.salary
WHERE old.name = "Bill"
DO INSTEAD
SELECT emp.salary
FROM emp
WHERE emp.name = "Joe";
</programlisting>
</para>
<para>
Deny Joe access to the salary of employees in the shoe
department (<function>current_user</function> returns the name of
the current user):
<programlisting>
CREATE RULE example_3 AS
ON
SELECT TO emp.salary
WHERE old.dept = "shoe" AND current_user = "Joe"
DO INSTEAD NOTHING;
</programlisting>
</para>
<para>
Create a view of the employees working in the toy department:
<programlisting>
CREATE toyemp(name = char16, salary = int4);
CREATE RULE example_4 AS
ON SELECT TO toyemp
DO INSTEAD
SELECT emp.name, emp.salary
FROM emp
WHERE emp.dept = "toy";
</programlisting>
</para>
<para>
All new employees must make 5,000 or less:
<programlisting>
CREATE RULE example_5 AS
ON INERT TO emp WHERE new.salary > 5000
DO
UPDATE emp SET salary = 5000
WHERE emp.oid = new.oid;
</programlisting>
</para>
</refsect1>
<refsect1 id="R1-SQL-CREATERULE-4">
<title>