Re: Querying distinct values from a large table

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