Re: Optimising "in" queries

["Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com>]

On 8/21/07, Stephen Davies <scldad(at)sdc(dot)com(dot)au> wrote:
> I have a PostgreSQL 8.2.4 table with some seven million rows.
>
> The psql query:
>
> select count(rdate),rdate from reading where sensor_id in
> (1137,1138,1139) group by rdate order by rdate desc limit 1;
>
> takes a few seconds but:
>
> select count(rdate),rdate from reading where sensor_id in
> (1137,1138,1139,1140) group by rdate order by rdate desc limit 1;
>
> (anything with four or more values in the "in" list) takes several
> minutes.

Can we see explain analyze output?  (i.e. not just plain explain)