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Diffstat (limited to 'doc/src/sgml/config.sgml')
| -rw-r--r-- | doc/src/sgml/config.sgml | 60 |
1 files changed, 51 insertions, 9 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 822bbf1f27..427947cf49 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -1690,22 +1690,64 @@ include_dir 'conf.d' </term> <listitem> <para> - Sets the maximum amount of memory to be used by a query operation + Sets the base maximum amount of memory to be used by a query operation (such as a sort or hash table) before writing to temporary disk files. If this value is specified without units, it is taken as kilobytes. The default value is four megabytes (<literal>4MB</literal>). Note that for a complex query, several sort or hash operations might be - running in parallel; each operation will be allowed to use as much memory - as this value specifies before it starts to write data into temporary - files. Also, several running sessions could be doing such operations - concurrently. Therefore, the total memory used could be many - times the value of <varname>work_mem</varname>; it is necessary to - keep this fact in mind when choosing the value. Sort operations are - used for <literal>ORDER BY</literal>, <literal>DISTINCT</literal>, and - merge joins. + running in parallel; each operation will generally be allowed + to use as much memory as this value specifies before it starts + to write data into temporary files. Also, several running + sessions could be doing such operations concurrently. + Therefore, the total memory used could be many times the value + of <varname>work_mem</varname>; it is necessary to keep this + fact in mind when choosing the value. Sort operations are used + for <literal>ORDER BY</literal>, <literal>DISTINCT</literal>, + and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of <literal>IN</literal> subqueries. </para> + <para> + Hash-based operations are generally more sensitive to memory + availability than equivalent sort-based operations. The + memory available for hash tables is computed by multiplying + <varname>work_mem</varname> by + <varname>hash_mem_multiplier</varname>. This makes it + possible for hash-based operations to use an amount of memory + that exceeds the usual <varname>work_mem</varname> base + amount. + </para> + </listitem> + </varlistentry> + + <varlistentry id="guc-hash-mem-multiplier" xreflabel="hash_mem_multiplier"> + <term><varname>hash_mem_multiplier</varname> (<type>floating point</type>) + <indexterm> + <primary><varname>hash_mem_multiplier</varname> configuration parameter</primary> + </indexterm> + </term> + <listitem> + <para> + Used to compute the maximum amount of memory that hash-based + operations can use. The final limit is determined by + multiplying <varname>work_mem</varname> by + <varname>hash_mem_multiplier</varname>. The default value is + 1.0, which makes hash-based operations subject to the same + simple <varname>work_mem</varname> maximum as sort-based + operations. + </para> + <para> + Consider increasing <varname>hash_mem_multiplier</varname> in + environments where spilling by query operations is a regular + occurrence, especially when simply increasing + <varname>work_mem</varname> results in memory pressure (memory + pressure typically takes the form of intermittent out of + memory errors). A setting of 1.5 or 2.0 may be effective with + mixed workloads. Higher settings in the range of 2.0 - 8.0 or + more may be effective in environments where + <varname>work_mem</varname> has already been increased to 40MB + or more. + </para> </listitem> </varlistentry> |