On Tue, Jul 4, 2017 at 9:23 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Aside from the mind-bendingly-tedious changes in pg_operator.h, the meat > of the patch is in selfuncs.c's ineq_histogram_selectivity(), which now > applies a correction for equal values in the cases where we were getting > it wrong before. While this logic seems experimentally correct (see > above), I have to admit that I've failed to wrap my brain around exactly > why it's correct. The arguments that I've constructed so far seem to > point in the direction of applying the opposite correction, which is > demonstrably wrong. Perhaps someone whose college statistics class > wasn't quite so long ago can explain this satisfactorily? > I guess that you're referring the last case, i.e. explain analyze select * from tenk1 where thousand between 10 and 10;
IMHO, following are the things that I've understood. The predicate internally got translated to predicate A (p >= 10) and predicate B (p <=10); In ineq_histogram_selectivity, For predicate A, hist_selec = p For predicate B, hist_selec = 1-p In clauselist_selectivity, we calculate the selectivity as = ((p) + (1 - p)) - 1= 0, rounded of to 1.0e-10. After your changes, In ineq_histogram_selectivity, For predicate A, hist_selec = p + correction (since, isgt=iseq) For predicate B, hist_selec = 1-p In clauselist_selectivity, we calculate the selectivity as = ((p + correction) + (1 - p)) - 1= correction, The correction is calculated as = 1 / num_distinct_values = .001. Since, the thousand column of tenk1 is uniformly distributed, this turns out to be the exact selectivity. (rows = .001 * 1000 = 10) Thoughts? -- Thanks & Regards, Kuntal Ghosh EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
