From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-performance@postgresql.org
Subject: Re: seq scan over 3.3 million rows instead of single key index access
Date: Sun, 23 Nov 2008 09:10:24 +0100
Message-ID: <20081123081024.GA7443@tux> (raw)
In-Reply-To: <gg9pg7$1edv$2@news.hub.org>
References: <gg9pg7$1edv$2@news.hub.org>
Andrus <kobruleht2@hot.ee> schrieb:
> There are indexes on rid(dokumnr) and dok(dokumnr) and dokumnr is int.
> Instead of using single key index, 8.1.4 scans over whole rid table.
> Sometimes idtelluued can contain more than single row so replacing join
> with equality is not possible.
>
> How to fix ?
>
> Andrus.
>
> CREATE TEMP TABLE idtellUued(dokumnr INT) ON COMMIT DROP;
> INSERT INTO idtellUued VALUES(1249228);
> explain analyze select 1
> from dok JOIN rid USING(dokumnr)
> JOIN idtellUued USING(dokumnr)
>
> "Hash Join (cost=7483.22..222259.77 rows=5706 width=0) (actual
> time=14905.981..27065.903 rows=8 loops=1)"
> " Hash Cond: ("outer".dokumnr = "inner".dokumnr)"
> " -> Seq Scan on rid (cost=0.00..198240.33 rows=3295833 width=4)
> (actual time=0.036..15021.641 rows=3280576 loops=1)"
How many rows contains rid? The estimation are okay, rows=3295833 and
actual rows=3280576 are nearly identical. An index-scan makes only sense
if rid contains considerable more than 3000000 rows.
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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To: pgsql-performance@postgresql.org
Cc: akretschmer@spamfence.net
Subject: Re: seq scan over 3.3 million rows instead of single key index access
In-Reply-To: <20081123081024.GA7443@tux>
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