Re: Slow query with sub-select

[Rick Genter <rick.genter@xxxxxxxxx>]

On Jul 16, 2011, at 4:14 PM, - - wrote:

I would like to count rows in q whose mid does not exist in t.

I would write such a query like this:

SELECT COUNT(*)

   FROM q

      LEFT OUTER JOIN t

         ON (t.mid = q.mid)

WHERE t.mid IS NULL;

And I would make sure there was an index on t.mid. (And for 9.2, as I understand it, q.mid as well, since I believe in 9.2 PostgreSQL will be able to compute the result strictly from the indexes without hitting the base tables.)

--

Rick Genter

rick.genter@xxxxxxxxx