Re: Problem with n_distinct being consistently inaccurate.

"Nick Fankhauser" <nickf(at)ontko(dot)com> writes:
> So the question is- how can I get a better estimate of n_distinct from
> analyze?
> If I alter the stats target as high as it will go, I get closer, but it
> still shows the index to be about 1/2 as selective as it actually is:

AFAIK, estimating number of distinct values from a small sample is
inherently an ill-conditioned problem. You should probably be happy
it can get within a factor of 2 ;-).

You could try sticking the correct n_distinct into pg_statistic by hand
just to see if it really does change the plan, but I'd like to think
that getting within a factor of 2 is good enough. If it's not, then we
probably ought to look for ways to avoid using number-of-distinct-values
statistics altogether, because we'll seldom have a hard value for it.

regards, tom lane