Re: Partitions and work_mem?

From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Dave Johansen
Sent: Wednesday, October 15, 2014 4:05 PM
To: Josh Berkus
Cc: pgsql-performance
Subject: Re: [PERFORM] Partitions and work_mem?

On Wed, Oct 15, 2014 at 10:10 AM, Josh Berkus <josh(at)agliodbs(dot)com<mailto:josh(at)agliodbs(dot)com>> wrote:
On 10/14/2014 10:08 AM, Dave Johansen wrote:
> I'm running Postgres 8.4 on RHEL 6 64-bit and I had a question about how
> work_mem and partitions interact.
>
> https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server#work_mem
> The above wiki states that "if a query involves doing merge sorts of 8
> tables, that requires 8 times work_mem." If I have a table that is
> partitioned does each partition count as a "table" and get its on work_mem?

In theory, this could happen. In practice, based on tests I did at Sun
with DBT3 and 8.3, no backend ever used more than 3X work_mem. This is
partly because the level of parallelism in postgres is extremely
limited, so we can't actually sort 8 partitions at the same time.

Thanks for the feedback. That's very helpful.

BTW, 8.4 is EOL. Maybe time to upgrade?

RHEL 6 isn't EOLed and we're working on moving to RHEL 7 but it's a slow process that will probably take quite a bit of time, if it ever happens.

Postgres 8.4 is EOL (RHEL).

Igor Neyman