Re: PoC/WIP: Extended statistics on expressions

On Wed, 17 Mar 2021 at 17:26, Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
> My concern is that the current behavior (where we prefer expression
> stats over multi-column stats to some extent) works fine as long as the
> parts are independent, but once there's dependency it's probably more
> likely to produce underestimates. I think underestimates for grouping
> estimates were a risk in the past, so let's not make that worse.
>

I'm not sure the current behaviour really is preferring expression
stats over multi-column stats. In this example, where we're grouping
by (a+b), (c+d) and have stats on [(a+b),c] and (c+d), neither of
those multi-column stats actually match more than one
column/expression. If anything, I'd go the other way and say that it
was wrong to use the [(a+b),c] stats in the first case, where they
were the only stats available, since those stats aren't really
applicable to (c+d), which probably ought to be treated as
independent. IOW, it might have been better to estimate the first case
as

ndistinct((a+b)) * ndistinct(c) * ndistinct(d)

and the second case as

ndistinct((a+b)) * ndistinct((c+d))

Regards,
Dean