Re: Optimising a query requiring seqscans=0

On Sep 14, 2006, at 11:15 AM, Russ Brown wrote:
> We recently upgraded our trac backend from sqlite to postgres, and I
> decided to have a little fun and write some reports that delve into
> trac's subversion cache, and got stuck with a query optimisation
> problem.
>
> Table revision contains 2800+ rows
> Table node_change contains 370000+.
<...>
> I've got stuck with this query:
>
>    SELECT author, COUNT(DISTINCT r.rev)
>      FROM revision AS r
> LEFT JOIN node_change AS nc
>        ON r.rev=nc.rev
>     WHERE r.time >= EXTRACT(epoch FROM (NOW() - interval '30
> days'))::integer

Man I really hate when people store time_t in a database...

>  GROUP BY r.author;
>
> Statistics are set to 20, and I have ANALYZEd both tables.
>
> The report itself isn't important, but I'm using this as an
> exercise in
> PostgreSQL query optimisation and planner tuning, so any help/hints
> would be appreciated.

Setting statistics higher (100-200), at least for the large table
will likely help. Also make sure that you've set effective_cache_size
correctly (I generally set it to total memory - 1G, assuming the
server has at least 4G in it).
--
Jim Nasby                                    jimn@enterprisedb.com
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)