Re: Querying distinct values from a large table

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