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Re: SQL - Indexing for performance on uniquness check...

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: "Loftis, Charles E" <charles(dot)loftis(at)eds(dot)com>,pgsql-novice(at)postgresql(dot)org
Subject: Re: SQL - Indexing for performance on uniquness check...
Date: 2004-07-19 04:05:48
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Lists: pgsql-novice
Josh Berkus <josh(at)agliodbs(dot)com> writes:
> Charles,
>> Sample query to return non-uniqueness
>> SELECT A1, A2, A3, ..., An
>> FROM Table
>> GROUP BY A1, A2, A3, ..., An
>> HAVING Count(*)>1

> In order for it to be even possible to use an index (a hashaggregate 
> operation, actually) on this table, you'd have to include *all* of the GROUP 
> BY columns in a single, multi-column index.

> However, it would be unlikely for PG to use any kind of an index in the 
> operation above, because of the number of columns, the unlikelyness of 
> grouping (i.e. there will only be a minority of rows with count(*) > 1) and 
> the fact that you're running this against the whole table.  So any
> kind of an index is liable to be useless.

Yeah.  If you are not expecting a huge number of groups, I think that it
would be more interesting to try a HashAggregate plan than a sort/group
plan.  For this you need 7.4 or later and a sort_mem setting large
enough to cover whatever the planner estimates the hashtable size to be.

			regards, tom lane

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