Message-ID: <4522CD8C.1000108@refractions.net>
Date: Tue, 03 Oct 2006 13:52:28 -0700
From: Graham Davis <gdavis@refractions.net>
Reply-To: gdavis@refractions.net
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To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Chris Browne <cbbrowne@acm.org>, pgsql-performance@postgresql.org
Subject: Re: BUG #2658: Query not using index
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>
	<4522C8D8.4010601@refractions.net> <10941.1159908489@sss.pgh.pa.us>
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Thanks Tom, that explains it and makes sense.  I guess I will have to 
accept this query taking 40 seconds, unless I can figure out another way 
to write it so it can use indexes.  If there are any more syntax 
suggestions, please pass them on.  Thanks for the help everyone.

Graham.


Tom Lane wrote:

>Graham Davis <gdavis@refractions.net> writes:
>  
>
>>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.
>>    
>>
>
>Not in a GROUP BY context, only for the simple case.  Per the comment in
>planagg.c:
>
>	 * We don't handle GROUP BY, because our current implementations of
>	 * grouping require looking at all the rows anyway, and so there's not
>	 * much point in optimizing MIN/MAX.
>
>The problem is that using an index to obtain the maximum value of ts for
>a given value of assetid is not the same thing as finding out what all
>the distinct values of assetid are.
>
>This could possibly be improved but it would take a considerable amount
>more work.  It's definitely not in the category of "bug fix".
>
>			regards, tom lane
>  
>


-- 
Graham Davis
Refractions Research Inc.
gdavis@refractions.net