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Update example of partially constraining join order to use a subselect

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in FROM instead of an auxiliary view.  We didn't have subselect-in-FROM
when I wrote this originally...
This commit is contained in:
Tom Lane 2001-02-19 00:24:30 +00:00
parent fa0cd643d2
commit a276392e52
1 changed files with 6 additions and 5 deletions
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11
doc/src/sgml/perform.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.1 2000/12/16 02:29:36 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.2 2001/02/19 00:24:30 tgl Exp $
-->
<chapter id="performance-tips">
@ -350,10 +350,11 @@ SELECT * FROM a CROSS JOIN b, c, d, e WHERE ...;
might not want to constrain the planner's search for a good ordering
of inner joins inside an outer join. You can't do that directly in the
JOIN syntax, but you can get around the syntactic limitation by using
views. For example,
subselects. For example,
<programlisting>
CREATE VIEW v1 AS SELECT ... FROM a, b, c WHERE ...;
SELECT * FROM d LEFT JOIN v1 ON (...);
SELECT * FROM d LEFT JOIN
(SELECT * FROM a, b, c WHERE ...) AS ss
ON (...);
</programlisting>
Here, joining D must be the last step in the query plan, but the
planner is free to consider various join orders for A,B,C.
@ -397,7 +398,7 @@ SELECT * FROM d LEFT JOIN v1 ON (...);
command, instead
of a series of INSERT commands. This reduces parsing, planning, etc
overhead a great deal. If you do this then it's not necessary to fool
around with autocommit.
around with autocommit, since it's only one command anyway.
</para>
</sect2>