From: Graham Davis <gdavis@refractions.net>
To: Chris Browne <cbbrowne@acm.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: BUG #2658: Query not using index
Date: Tue, 03 Oct 2006 13:32:24 -0700
Message-ID: <4522C8D8.4010601@refractions.net> (raw)
In-Reply-To: <60d599gqcz.fsf@dba2.int.libertyrms.com>
References: <200609272056.k8RKuWjY040283@wwwmaster.postgresql.org>
<20061003000147.GA31475@wolff.to>
<4522AA01.4020709@refractions.net>
<60irj1gr3r.fsf@dba2.int.libertyrms.com>
<4522B5B9.2050206@refractions.net>
<60d599gqcz.fsf@dba2.int.libertyrms.com>
How come an aggreate like that has to use a sequential scan? I know
that PostgreSQL use to have to do a sequential scan for all aggregates,
but there was support added to version 8 so that aggregates would take
advantage of indexes. This is why
SELECT max(ts) AS ts
FROM asset_positions;
Uses an index on the ts column and only takes 50 milliseconds. When I
added the group by it would not use a multikey index or any other
index. Is there just no support for aggregates to use multikey
indexes? Sorry to be so pushy, but I just want to make sure I
understand why the above query can use an index and the following can't:
SELECT assetid, max(ts) AS ts
FROM asset_positions
GROUP BY assetid;
--
Graham Davis
Refractions Research Inc.
gdavis@refractions.net
Chris Browne wrote:
>gdavis@refractions.net (Graham Davis) writes:
>
>
>>40 seconds is much too slow for this query to run and I'm assuming
>>that the use of an index will make it much faster (as seen when I
>>removed the GROUP BY clause). Any tips?
>>
>>
>
>Assumptions are dangerous things.
>
>An aggregate like this has *got to* scan the entire table, and given
>that that is the case, an index scan is NOT optimal; a seq scan is.
>
>An index scan is just going to be slower.
>
>
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To: pgsql-performance@postgresql.org
Cc: gdavis@refractions.net, cbbrowne@acm.org
Subject: Re: BUG #2658: Query not using index
In-Reply-To: <4522C8D8.4010601@refractions.net>
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