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Diffstat (limited to 'doc/src/sgml/config.sgml')
| -rw-r--r-- | doc/src/sgml/config.sgml | 43 |
1 files changed, 10 insertions, 33 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 2d1e508183..8322463cea 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -1,5 +1,5 @@ <!-- -$PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.38 2005/12/09 15:51:13 petere Exp $ +$PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.39 2005/12/20 02:30:35 tgl Exp $ --> <chapter Id="runtime-config"> <title>Server Configuration</title> @@ -2028,6 +2028,7 @@ SELECT * FROM parent WHERE key = 2400; 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 <xref linkend="guc-geqo-threshold">. + For more information see <xref linkend="explicit-joins">. </para> </listitem> </varlistentry> @@ -2039,48 +2040,24 @@ SELECT * FROM parent WHERE key = 2400; </indexterm> <listitem> <para> - 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 parameter 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> + The planner will rewrite explicit <literal>JOIN</> + constructs (except <literal>FULL JOIN</>s) into lists of + <literal>FROM</> items whenever a list of no more than this many items + would result. Smaller values reduce planning time but may + yield inferior query plans. </para> <para> By default, this variable is set the same as <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 + explicit <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). + order they desire explicitly. + For more information see <xref linkend="explicit-joins">. </para> </listitem> </varlistentry> |