Re: Risk Estimation WAS: Planner hints in Postgresql

Robert Haas <robertmhaas@gmail.com> writes:
> So you might think that the problem here is that we're assuming
> uniform density.  Let's say there are a million rows in the table, and
> there are 100 that match our criteria, so the first one is going to
> happen 1/10,000'th of the way through the table.  Thus we set SC =
> 0.0001 * TC, and that turns out to be an underestimate if the
> distribution isn't as favorable as we're hoping.  However, that is NOT
> what we are doing.  What we are doing is setting SC = 0.  I mean, not
> quite 0, but yeah, effectively 0. Essentially we're assuming that no
> matter how selective the filter condition may be, we assume that it
> will match *the very first row*.

I think this is wrong.  Yeah, the SC may be 0 or near it, but the time to
fetch the first tuple is estimated as SC + (TC-SC)/N.
        regards, tom lane