| From: | "Vladimir Sitnikov" <sitnikov(dot)vladimir(at)gmail(dot)com> |
|---|---|
| To: | "Richard Huxton" <dev(at)archonet(dot)com> |
| Cc: | firerox(at)centrum(dot)cz, tv(at)fuzzy(dot)cz, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: slow full table update |
| Date: | 2008-11-12 17:58:35 |
| Message-ID: | 1d709ecc0811120958k5be6235vc3dbeb6fb58f4a68@mail.gmail.com |
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| Lists: | pgsql-performance |
> >
> > Recheck Cond: ((sid > 20000) AND (sid < 30000))
> >
> > -> Bitmap Index Scan on pk_songs2 (cost=0.00..151.59 rows=8931
> width=0) (actual time=4.071..4.071 rows=9579 loops=1)
> >
> > Index Cond: ((sid > 20000) AND (sid < 30000))
> >
> > Is there a way to run this query on sigle throughpass with no Recheck
> Cond?
>
> "Recheck Cond" is somewhat misleading here.
Bitmap Index Scan has almost void "recheck" impact in case the whole bitmap
fits in work_mem. That means bitmap scan degrades when the number of rows in
table (not the total number of returned rows) is greater than
work_mem*1024*8. 60'000 rows bitmap scan will require 60'000/8=7'500 bytes ~
8Kbytes of memory to run without additional recheck, thus I do not believe
it hurts you in this particular case
Regards,
Vladimir Sitnikov
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