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Improve the documentation of the 'join_collapse_limit' GUC var. Thanks to

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Tom Lane for some editorial suggestions.
This commit is contained in:
Neil Conway 2004-01-27 16:51:43 +00:00
parent 7e34b2d73f
commit 45842e6c52
1 changed files with 52 additions and 17 deletions
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69
doc/src/sgml/runtime.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.234 2004/01/25 03:30:00 momjian Exp $
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.235 2004/01/27 16:51:43 neilc Exp $
-->
<Chapter Id="runtime">
@ -1462,11 +1462,11 @@ SET ENABLE_SEQSCAN TO OFF;
<term><varname>from_collapse_limit</varname> (<type>integer</type>)</term>
<listitem>
<para>
The planner will merge sub-queries into upper queries if the resulting
FROM list would have no more than this many items. Smaller values
reduce planning time but may yield inferior query plans.
The default is 8. It is usually wise to keep this less than
<varname>geqo_threshold</varname>.
The planner will merge sub-queries into upper queries if the
resulting <literal>FROM</literal> list would have no more than
this many items. Smaller values reduce planning time but may
yield inferior query plans. The default is 8. It is usually
wise to keep this less than <varname>geqo_threshold</varname>.
</para>
</listitem>
</varlistentry>
@ -1475,14 +1475,48 @@ SET ENABLE_SEQSCAN TO OFF;
<term><varname>join_collapse_limit</varname> (<type>integer</type>)</term>
<listitem>
<para>
The planner will flatten explicit inner <literal>JOIN</> constructs
into lists of <literal>FROM</> items whenever a list of no more than
this many items would result. Usually this is set the same as
<varname>from_collapse_limit</>. Setting it to 1 prevents any
flattening of inner <literal>JOIN</>s, allowing explicit
<literal>JOIN</> syntax to be used to control the join order.
Intermediate values might be useful to trade off planning time
against quality of plan.
The planner will rewrite explicit inner <literal>JOIN</>
constructs into lists of <literal>FROM</> items whenever a
list of no more than this many items in total would
result. Prior to <productname>PostgreSQL</> 7.4, joins
specified via the <literal>JOIN</literal> construct would
never be reordered by the query planner. The query planner has
subsequently been improved so that inner joins written in this
form can be reordered; this configuration variable controls
the extent to which this reordering is performed.
<note>
<para>
At present, the order of outer joins specified via the
<literal>JOIN</> construct is never adjusted by the query
planner; therefore, <varname>join_collapse_limit</> has no
effect on this behavior. The planner may be improved to
reorder some classes of outer joins in a future release of
<productname>PostgreSQL</productname>.
</para>
</note>
</para>
<para>
By default, this variable is set to
<varname>from_collapse_limit</varname>, which is appropriate
for most uses. Setting it to 1 prevents any reordering of
inner <literal>JOIN</>s. Thus, the explicit join order
specified in the query will be the actual order in which the
relations are joined. The query planner does not always choose
the optimal join order; advanced users may elect to
temporarily set this variable to 1, and then specify the join
order they desire explicitly. Another consequence of setting
this variable to 1 is that the query planner will behave more
like the <productname>PostgreSQL</productname> 7.3 query
planner, which some users might find useful for backward
compatibility reasons.
</para>
<para>
Setting this variable to a value between 1 and
<varname>from_collapse_limit</varname> might be useful to
trade off planning time against the quality of the chosen plan
(higher values produce better plans).
</para>
</listitem>
</varlistentry>
@ -2583,9 +2617,10 @@ dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
<term><varname>max_identifier_length</varname> (<type>integer</type>)</term>
<listitem>
<para>
Shows the maximum identifier length. It is determined as one less than
the value of <literal>NAMEDATALEN</> when building the server. The
default value of NAMEDATALEN is 64; therefore the default
Shows the maximum identifier length. It is determined as one
less than the value of <literal>NAMEDATALEN</> when building
the server. The default value of <literal>NAMEDATALEN</> is
64; therefore the default
<varname>max_identifier_length</varname> is 63.
</para>
</listitem>