>> 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
