Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB - Mailing list pgsql-general

From Andreas Kretschmer
Subject Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB
Date
Msg-id 20110222171740.GA6399@tux
Whole thread Raw
In response to Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-general
Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Reid Thompson <Reid.Thompson@ateb.com> writes:
> > What am I missing that causes this to resort to sorting on disk?
>
> The in-memory space required to sort N tuples can be significantly
> larger than the on-disk space, because the latter representation is
> optimized to be small and the in-memory representation not so much.
> I haven't seen a 3X differential before, but it's not outside the realm
> of reason, especially for narrow rows like these where it's all about
> the overhead.  I suspect if you crank work_mem up still more, you'll see
> it switch over.  It flips to on-disk sort when the in-memory
> representation exceeds the limit ...

Question: when is the planner making the decision between in-memory and
on-disk, at planning-time or at execution time with the knowledge about
the real amount of tuples?


Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

pgsql-general by date:

Previous
From: David Kerr
Date:
Subject: Re: Reordering a table
Next
From: Vibhor Kumar
Date:
Subject: Re: Reordering a table