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Re: Querying distinct values from a large table
- From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
- To: "Igor Lobanov" <ilobanov(at)swsoft(dot)com>
- Cc: "Richard Huxton" <dev(at)archonet(dot)com>, <pgsql-performance(at)postgresql(dot)org>
- Subject: Re: Querying distinct values from a large table
- Date: Tue, 30 Jan 2007 14:04:15 +0000
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. -- Simon Riggs EnterpriseDB http://www.enterprisedb.com
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