| From: | Richard Huxton <dev(at)archonet(dot)com> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Igor Lobanov <ilobanov(at)swsoft(dot)com>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Querying distinct values from a large table |
| Date: | 2007-01-30 14:11:48 |
| Message-ID: | 45BF5224.3070805@archonet.com |
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| Lists: | pgsql-performance |
Simon Riggs wrote:
> On Tue, 2007-01-30 at 15:33 +0600, Igor Lobanov wrote:
>
>> explain analyze select distinct a, b from tbl
>>
>> EXPLAIN ANALYZE output is:
>>
>> Unique (cost=500327.32..525646.88 rows=1848 width=6) (actual
>> time=52719.868..56126.356 rows=5390 loops=1)
>> -> Sort (cost=500327.32..508767.17 rows=3375941 width=6) (actual
>> time=52719.865..54919.989 rows=3378864 loops=1)
>> Sort Key: a, b
>> -> Seq Scan on tbl (cost=0.00..101216.41 rows=3375941
>> width=6) (actual time=16.643..20652.610 rows=3378864 loops=1)
>> Total runtime: 57307.394 ms
>
> All your time is in the sort, not in the SeqScan.
>
> Increase your work_mem.
Well, even if the sort was instant it's only going to get him down to
20secs.
--
Richard Huxton
Archonet Ltd
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