Hi,
The attached patch version modifies how the non-MCV selectivity is
computed, along the lines explained in the previous message.
The comments in statext_clauselist_selectivity() explain it in far more
detail, but we this:
1) Compute selectivity using the MCV (s1).
2) To compute the non-MCV selectivity (s2) we do this:
2a) See how many top-level equalities are there (and compute ndistinct
estimate for those attributes).
2b) If there is an equality on each column, we know there can only be a
single matching item. If we found it in the MCV (i.e. s1 > 0) we're
done, and 's1' is the answer.
2c) If only some columns have equalities, we estimate the selectivity
for equalities as
s2 = ((1 - mcv_total_sel) / ndistinct)
If there are no remaining conditions, we're done.
2d) To estimate the non-equality clauses (on non-MCV part only), we
either repeat the whole process by calling clauselist_selectivity() or
approximating s1 to the non-MCV part. This needs a bit of care to
prevent infinite loops.
Of course, with 0002 this changes slightly, because we may try using a
histogram to estimate the non-MCV part. But that's just an extra step
right before (2a).
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
