>> If we query on values that aren't in the table, the planner will >> always overestimate the expected number of returned rows because it ( >> implicitly ) assumes that every query will return at least 1 record. > > That's intentional and should not be changed.
Why? What if ( somehow ) we knew that there was a 90% chance that query would return an empty result set on a big table with 20 non-mcv distinct values. Currently the planner would always choose a seq scan, where an index scan might be better. Better yet, couldn't that be optimized to *if record exists, execute seq scan*. That being said, I think queries are generally searching for values that exist in the table. > I can't see the value of allowing fractional-row estimates anyway. Neither can I, but I could probably think of cases where knowing the SD of the result set could result in better plans. -Nathan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
