agora inbox for pgsql-admin@postgresql.org
help / color / mirror / Atom feedFrom: Paul Smith* <paul@pscs.co.uk>
To: pgsql-admin@lists.postgresql.org
Subject: Re: small temp files
Date: Mon, 22 Jul 2024 14:34:09 +0100
Message-ID: <fe4b82fa-9907-43f9-b6ff-e1cbc768158d@pscs.co.uk> (raw)
In-Reply-To: <8E03B792-C389-456D-913D-4F2B0EBB903E@elevated-dev.com>
References: <7A0C9A69-5632-4CFC-B156-53FAAAB33E26@elevated-dev.com>
<a9dee6e8-6197-d9bb-028e-6d020167bf57@jakobs.com>
<8E03B792-C389-456D-913D-4F2B0EBB903E@elevated-dev.com>
On 22/07/2024 14:28, Scott Ribe wrote:
> I understand those things--my question is why, with work_mem set to 128MB, I would see tiny temp files (7452 is common, as is 102, and I've seen as small as 51).
>
From the manual:
"Note that a complex query might perform several sort and hash
operations at the same time, with each operation generally being 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|work_mem|; it is necessary to keep this fact in
mind when choosing the value. Sort operations are used for|ORDER
BY|,|DISTINCT|, and merge joins. Hash tables are used in hash joins,
hash-based aggregation, memoize nodes and hash-based processing
of|IN|subqueries."
So, if it's doing lots of joins, there may be lots of bits of temporary
data which together add up to more than work_mem. AIUI PostgreSQL
doesn't necessarily shove all the temporary data for one query into one
file, it may have multiple smaller files for the data for a single query.
view thread (8+ messages) latest in thread
Message-ID: <fe4b82fa-9907-43f9-b6ff-e1cbc768158d@pscs.co.uk>
Permalink: ../fe4b82fa-9907-43f9-b6ff-e1cbc768158d@pscs.co.uk/
Also on: postgresql.org/message-id/fe4b82fa-9907-43f9-b6ff-e1cbc768158d@pscs.co.uk
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-admin@postgresql.org
Cc: paul@pscs.co.uk, pgsql-admin@lists.postgresql.org
Subject: Re: small temp files
In-Reply-To: <fe4b82fa-9907-43f9-b6ff-e1cbc768158d@pscs.co.uk>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox