> 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.
Sounds like an opportunity to implement a "Sort Unique" (sort of like a hash, I guess), there is no need to push 3M rows through a sort algorithm to only shave it down to 1848 unique records.
I am assuming this optimization just isn't implemented in PostgreSQL?
--
Chad
http://www.postgresqlforums.com/
