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Commit917bbeb

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Apply a simple solution to the problem of making INSERT/UPDATE/DELETE
RETURNING play nice with views/rules. To wit, have the rule rewriterrewrite any RETURNING clause found in a rule to produce what the rule'striggering query asked for in its RETURNING clause, in particular dropthe RETURNING clause if no RETURNING in the triggering query. Thisleaves the responsibility for knowing how to produce the view's outputcolumns on the rule author, without requiring any fundamental changesin rule semantics such as adding new rule event types would do. Theinitial implementation constrains things to ensure that there isexactly one, unconditionally invoked RETURNING clause among the rulesfor an event --- later we might be able to relax that, but for a postfeature freeze fix it seems better to minimize how much invention we do.Per gripe from Jaime Casanova.
1 parent74924d2 commit917bbeb

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6 files changed

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6 files changed

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‎doc/src/sgml/ref/create_rule.sgml

Lines changed: 24 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
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<!--
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$PostgreSQL: pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.45 2005/01/04 00:39:53 tgl Exp $
2+
$PostgreSQL: pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.46 2006/09/02 17:06:52 tgl Exp $
33
PostgreSQL documentation
44
-->
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@@ -70,7 +70,9 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
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<literal>ON INSERT</literal>, <literal>ON UPDATE</literal>, and
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<literal>ON DELETE</literal> rules (or any subset of those that's
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sufficient for your purposes) to replace update actions on the view
73-
with appropriate updates on other tables.
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with appropriate updates on other tables. If you want to support
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<command>INSERT RETURNING</> and so on, then be sure to put a suitable
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<literal>RETURNING</> clause into each of these rules.
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</para>
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<para>
@@ -87,7 +89,8 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
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understands it will never be called on to update the dummy table.
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Then make the conditional rules non-<literal>INSTEAD</literal>; in
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the cases where they are applied, they add to the default
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<literal>INSTEAD NOTHING</literal> action.
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<literal>INSTEAD NOTHING</literal> action. (This method does not
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currently work to support <literal>RETURNING</> queries, however.)
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</para>
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</refsect1>
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@@ -201,13 +204,30 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
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be allowed to define a rule on it.
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</para>
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<para>
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In a rule for <literal>INSERT</literal>, <literal>UPDATE</literal>, or
209+
<literal>DELETE</literal> on a view, you can add a <literal>RETURNING</>
210+
clause that emits the view's columns. This clause will be used to compute
211+
the outputs if the rule is triggered by an <command>INSERT RETURNING</>,
212+
<command>UPDATE RETURNING</>, or <command>DELETE RETURNING</> command
213+
respectively. When the rule is triggered by a command without
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<literal>RETURNING</>, the rule's <literal>RETURNING</> clause will be
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ignored. The current implementation allows only unconditional
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<literal>INSTEAD</> rules to contain <literal>RETURNING</>; furthermore
217+
there can be at most one <literal>RETURNING</> clause among all the rules
218+
for the same event. (This ensures that there is only one candidate
219+
<literal>RETURNING</> clause to be used to compute the results.)
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<literal>RETURNING</> queries on the view will be rejected if
221+
there is no <literal>RETURNING</> clause in any available rule.
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</para>
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<para>
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It is very important to take care to avoid circular rules. For
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example, though each of the following two rule definitions are
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accepted by <productname>PostgreSQL</productname>, the
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<command>SELECT</command> command would cause
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<productname>PostgreSQL</productname> to report an error because
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the query cycled too many times:
230+
of recursive expansion of a rule:
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<programlisting>
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CREATE RULE "_RETURN" AS

‎doc/src/sgml/rules.sgml

Lines changed: 36 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,4 @@
1-
<!-- $PostgreSQL: pgsql/doc/src/sgml/rules.sgml,v 1.45 2006/04/23 03:39:52momjian Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/rules.sgml,v 1.46 2006/09/02 17:06:52tgl Exp $ -->
22

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<chapter id="rules">
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<title>The Rule System</title>
@@ -873,7 +873,7 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
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<listitem>
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<para>
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They can be <literal>INSTEAD</> or <literal>ALSO</> (default).
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They can be <literal>INSTEAD</> or <literal>ALSO</> (thedefault).
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</para>
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</listitem>
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@@ -920,7 +920,8 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
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Initially the query-tree list is empty.
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There can be zero (<literal>NOTHING</> key word), one, or multiple actions.
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To simplify, we will look at a rule with one action. This rule
923-
can have a qualification or not and it can be <literal>INSTEAD</> or <literal>ALSO</> (default).
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can have a qualification or not and it can be <literal>INSTEAD</> or
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<literal>ALSO</> (the default).
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</para>
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<para>
@@ -932,22 +933,13 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
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</para>
933934

934935
<para>
935-
So we havefour cases that produce the following query trees for
936+
So we havethree cases that produce the following query trees for
936937
a one-action rule.
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938939
<variablelist>
939940
<varlistentry>
940-
<term>No qualification and <literal>ALSO</></term>
941-
<listitem>
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<para>
943-
the query tree from the rule action with the original query
944-
tree's qualification added
945-
</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>No qualification but <literal>INSTEAD</></term>
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<term>No qualification, with either <literal>ALSO</> or
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<literal>INSTEAD</></term>
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<listitem>
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<para>
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the query tree from the rule action with the original query
@@ -1283,7 +1275,7 @@ SELECT shoelace_data.sl_name, 0,
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A simple way to protect view relations from the mentioned
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possibility that someone can try to run <command>INSERT</command>,
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<command>UPDATE</command>, or <command>DELETE</command> on them is
1286-
to let those query trees get thrown away. So we create the rules
1278+
to let those query trees get thrown away. So wecouldcreate the rules
12871279

12881280
<programlisting>
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CREATE RULE shoe_ins_protect AS ON INSERT TO shoe
@@ -1338,6 +1330,34 @@ CREATE RULE shoelace_del AS ON DELETE TO shoelace
13381330
</programlisting>
13391331
</para>
13401332

1333+
<para>
1334+
If you want to support <literal>RETURNING</> queries on the view,
1335+
you need to make the rules include <literal>RETURNING</> clauses that
1336+
compute the view rows. This is usually pretty trivial for views on a
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single table, but it's a bit tedious for join views such as
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<literal>shoelace</literal>. An example for the insert case is
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1340+
<programlisting>
1341+
CREATE RULE shoelace_ins AS ON INSERT TO shoelace
1342+
DO INSTEAD
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INSERT INTO shoelace_data VALUES (
1344+
NEW.sl_name,
1345+
NEW.sl_avail,
1346+
NEW.sl_color,
1347+
NEW.sl_len,
1348+
NEW.sl_unit
1349+
)
1350+
RETURNING
1351+
shoelace_data.*,
1352+
(SELECT shoelace_data.sl_len * u.un_fact
1353+
FROM unit u WHERE shoelace_data.sl_unit = u.un_name);
1354+
</programlisting>
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1356+
Note that this one rule supports both <command>INSERT</> and
1357+
<command>INSERT RETURNING</> queries on the view &mdash; the
1358+
<literal>RETURNING</> clause is simply ignored for <command>INSERT</>.
1359+
</para>
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13411361
<para>
13421362
Now assume that once in a while, a pack of shoelaces arrives at
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the shop and a big parts list along with it. But you don't want

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