On Mon, Mar 26, 2012 at 5:43 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Hm. This illustrates that it's not too prudent to rely on a default
> numdistinct estimate to decide that a hash aggregation is safe :-(.
> We had probably better tweak the cost estimation rules to not trust
> that. Maybe, if we have a default estimate, we should take the worst
> case estimate that the column might be unique? That could still burn
> us if the rowcount estimate was horribly wrong, but those are not nearly
> as shaky as numdistinct estimates ...
The selectivity API should include some way of indicating the accuracy
of the answer, as well as the answer itself.
That way we could respond better in a wide range of circumstances.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
