</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>

The new approach makes better use of the <acronym>CPU</> cache

for typical cache sizes and data volumes. Where necessary,

the behavior can be adjusted via the new configuration parameter

<xref linkend="guc-replacement-sort-tuples">.

<literal>replacement_sort_tuples</literal>.

</para>

</listitem>

boolallowSystemTableMods= false;

intwork_mem=1024;

intmaintenance_work_mem=16384;

intreplacement_sort_tuples=150000;

NULL,NULL,NULL

},

{

{"replacement_sort_tuples",PGC_USERSET,RESOURCES_MEM,

gettext_noop("Sets the maximum number of tuples to be sorted using replacement selection."),

gettext_noop("When more tuples than this are present, quicksort will be used.")

},

&replacement_sort_tuples,

150000,0,INT_MAX,

NULL,NULL,NULL

},

# you actively intend to use prepared transactions.

#work_mem = 4MB# min 64kB

#maintenance_work_mem = 64MB# min 1MB

#replacement_sort_tuples = 150000# limits use of replacement selection sort

#autovacuum_work_mem = -1# min 1MB, or -1 to use maintenance_work_mem

#max_stack_depth = 2MB# min 100kB

#dynamic_shared_memory_type = posix# the default is the first option