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Re: Problem with bitmap-index-scan plan

From: Kapadaidakis Yannis <jkapad(at)csd(dot)uoc(dot)gr>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Problem with bitmap-index-scan plan
Date: 2006-07-18 09:25:44
Message-ID: Pine.GSO.4.64.0607181224540.11316@oneiro.csd.uoc.gr (view raw or flat)
Thread:
Lists: pgsql-performance
Yes that was the problem! Thank you very much

On Thu, 13 Jul 2006, Tom Lane wrote:

> jkapad(at)csd(dot)uoc(dot)gr writes:
>> ... is quite reasonable.The table has 1.000.000 rows (17.242 pages). From
>> pg_stat_get_blocks_fetched I can see that there were 102 page requests for
>> table. So all things seem to work great here!
>
>> But if I multiply the size of the table ten-times (10.000.000 rows - 172.414
>> pages) and run the same query I get:
>> ...
>> which is slower even than a seq scan. Now I get that there were 131.398 page
>> requests for table in order to retrieve almost 1250 tuples!Can someone explain
>> why this is happening? All memory parameters are set to default.
>
> You probably need to increase work_mem so that the bitmaps don't become
> lossy ...
>
> 			regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: In versions below 8.0, the planner will ignore your desire to
>       choose an index scan if your joining column's datatypes do not
>       match
>

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