> From: Daulat Ram <Daulat.Ram@cyient.com>
> To: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
> Sent: Thursday, 13 April 2017, 7:25
> Subject: [PERFORM] Hi
>
> Hello,
>
> I need to know the criteria behind for settings the work_mem in PostgreSQL, please give the example also if possible.
>
> Regards,
> Daulat
Is there anything in particular from the manual pages you don't understand? It should be quite clear:
https://www.postgresql.org/docs/current/static/runtime-config-resource.html
"Specifies the amount of memory to be used by internal sort operations and hash tables before writing to temporary disk
files.The value defaults to four megabytes (4MB). Note that for a complex query, several sort or hash operations might
berunning in parallel; each operation will be allowed to use as much memory as this value specifies before it starts to
writedata into temporary files."
"Also, several running sessions could be doing such operations concurrently. Therefore, the total memory used could be
manytimes the value of work_mem; it is necessary to keep this fact in mind when choosing the value. Sort operations are
usedfor ORDER BY, DISTINCT, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based
processingof IN subqueries."
Glyn
