> Obviously we run into problems when
> a) we have a poor estimate for ndistinct - but then we have
> worse problems
> b) our length measure doesn't correspond well with ndistinct
> in an interval
One more problem with low ndistinct values is that the condition might very well
hit no rows at all. But Idea 1 will largely overestimate the number of hits.
e.g. char(2) field has a histogram bin for 'a1' - 'b1' ndistinct is 2 because actual
values in the bin are 'a1' and 'a2'. A query for 'a3' now has a bogus estimate of nrowsperbin / 2.
I think for low ndistinct values we will want to know the exact
value + counts and not a bin. So I think we will want additional stats rows
that represent "value 'a1' stats".
Andreas
