Re: Querying distinct values from a large table

On 1/30/07, Simon Riggs <simon@2ndquadrant.com> 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.
>

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/