Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB
От | Andreas Kretschmer |
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Тема | Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB |
Дата | |
Msg-id | 20110222171740.GA6399@tux обсуждение исходный текст |
Ответ на | Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB
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Список | 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°
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