From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Guillaume Cottenceau <gc@mnc.ch>
Cc: pgsql-performance@postgresql.org
Subject: Re: Poor performance on seq scan
Date: Tue, 12 Sep 2006 14:24:24 +0100
Message-ID: <4506B508.80806@enterprisedb.com> (raw)
In-Reply-To: <87y7spw8v9.fsf@meuh.mnc.lan>
References: <450684EC.4090304@designaproduct.biz>
<4506902C.7090407@enterprisedb.com>
<4506A9E7.2060303@designaproduct.biz>
<87y7spw8v9.fsf@meuh.mnc.lan>
Guillaume Cottenceau wrote:
> Laszlo Nagy <gandalf 'at' designaproduct.biz> writes:
>
>>> Probably, but PostgreSQL doesn't know how to do that. Even if it
>>> did, it depends on how many matches there is. If you scan the index
>>> and then fetch the matching rows from the heap, you're doing random
>>> I/O to the heap. That becomes slower than scanning the heap
>>> sequentially if you're going to get more than a few hits.
>>>
>> I have 700 000 rows in the table, and usually there are less than 500
>> hits. So probably using a "seq index scan" would be faster. :-) Now I
>>
>
> You can confirm this idea by temporarily disabling sequential
> scans. Have a look at this chapter:
>
I don't think it will anyway do a "seq index scan" as Laszlo envisions.
PostgreSQL cannot do "fetch index tuple and apply %Mug% to it. If it
matches, fetch heap tuple". Even if you disable sequential scans, it's
still going to fetch every heap tuple to see if it matches "%Mug%". It's
just going to do it in index order, which is slower than a seq scan.
BTW: in addition to setting enable_seqscan=false, you probably have to
add a dummy where-clause like "name > ''" to force the index scan.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-performance@postgresql.org
Cc: heikki@enterprisedb.com, gc@mnc.ch
Subject: Re: Poor performance on seq scan
In-Reply-To: <4506B508.80806@enterprisedb.com>
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