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Diffstat (limited to 'doc/src/sgml/config.sgml')
| -rw-r--r-- | doc/src/sgml/config.sgml | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index fdeda90f03d..2f04702b6e5 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -1472,6 +1472,45 @@ include_dir 'conf.d' </listitem> </varlistentry> + <varlistentry id="guc-replacement-sort-tuples" xreflabel="replacement_sort_tuples"> + <term><varname>replacement_sort_tuples</varname> (<type>integer</type>) + <indexterm> + <primary><varname>replacement_sort_tuples</> configuration parameter</primary> + </indexterm> + </term> + <listitem> + <para> + When the number of tuples to be sorted is smaller than this number, + a sort will produce its first output run using replacement selection + rather than quicksort. This may be useful in memory-constrained + environments where tuples that are input into larger sort operations + have a strong physical-to-logical correlation. Note that this does + not include input tuples with an <emphasis>inverse</emphasis> + correlation. It is possible for the replacement selection algorithm + to generate one long run that requires no merging, where use of the + default strategy would result in many runs that must be merged + to produce a final sorted output. This may allow sort + operations to complete sooner. + </para> + <para> + The default is 150,000 tuples. Note that higher values are typically + not much more effective, and may be counter-productive, since the + priority queue is sensitive to the size of available CPU cache, whereas + the default strategy sorts runs using a <firstterm>cache + oblivious</firstterm> algorithm. This property allows the default sort + strategy to automatically and transparently make effective use + of available CPU cache. + </para> + <para> + Setting <varname>maintenance_work_mem</varname> to its default + value usually prevents utility command external sorts (e.g., + sorts used by <command>CREATE INDEX</> to build B-Tree + indexes) from ever using replacement selection sort, unless the + input tuples are quite wide. + </para> + </listitem> + </varlistentry> + <varlistentry id="guc-autovacuum-work-mem" xreflabel="autovacuum_work_mem"> <term><varname>autovacuum_work_mem</varname> (<type>integer</type>) <indexterm> |