| From: | "Chad Wagner" <chad(dot)wagner(at)gmail(dot)com> |
|---|---|
| To: | "Simon Riggs" <simon(at)2ndquadrant(dot)com> |
| Cc: | "Igor Lobanov" <ilobanov(at)swsoft(dot)com>, "Richard Huxton" <dev(at)archonet(dot)com>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Querying distinct values from a large table |
| Date: | 2007-01-30 14:13:27 |
| Message-ID: | 81961ff50701300613g25ea4ce6jb357c82fb1ed6733@mail.gmail.com |
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| Thread: | |
| Lists: | pgsql-performance |
On 1/30/07, Simon Riggs <simon(at)2ndquadrant(dot)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?
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