Tomas Vondra <tv(at)fuzzy(dot)cz> writes:
> AFAIK it will work with reasonably precise estimates, but the point is
> you need an estimate of distinct values of the whole group of columns.
> So when you want to get an estimate for queries on columns (a,b), you
> need the number of distinct value combinations of these two columns.
That seems likely to be even more unreliable than our existing
single-column estimates :-(
regards, tom lane