| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
| Cc: | David Wilson <david(dot)t(dot)wilson(at)gmail(dot)com>, Alban Hertroys <dalroi(at)solfertje(dot)student(dot)utwente(dot)nl>, pgsql-general(at)postgresql(dot)org |
| Subject: | Re: select distinct and index usage |
| Date: | 2008-04-08 02:30:31 |
| Message-ID: | 13767.1207621831@sss.pgh.pa.us |
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| Lists: | pgsql-general |
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Tom Lane escribi:
>> What I think you'll find, though, is that once you do force an indexscan
>> to be picked it'll be slower. Full-table index scans are typically
>> worse than seqscan+sort, unintuitive though that may sound.
> Hmm, should we switch the CLUSTER code to do that?
It's been suggested before, but I'm not sure. The case where an
indexscan can win is where the table is roughly in index order already.
So if you think about periodic CLUSTER to maintain table ordering,
I suspect you'd want the indexscan implementation for all but maybe
the first time.
regards, tom lane
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