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.)