Improve eqjoinsel_semi's behavior for small inner relations with no stats. If we don't have any MCV statistics for the inner relation, and we don't trust its numdistinct estimate either, eqjoinsel_semi falls back to a very conservative estimate (that 50% of the outer rows have matches). This is particularly problematic if the inner relation is completely empty, since then even an explicit ANALYZE won't produce any pg_statistic entries, so there's no way to budge the planner off the bad estimate.
We'd produce a better estimate in such cases if we used the nd2/nd1 selectivity heuristic, so an easy fix is to treat the nd2 estimate as non-default if we derive it from clamping to the inner rel's rowcount estimate. This won't fix every related case (mainly because the rowcount estimate might be larger than DEFAULT_NUM_DISTINCT), but it seems like a sane extension of the existing logic, so let's apply the change in HEAD and see if anyone complains. Per bug #14438 from Nikolay Nikitin. Report: https://postgr.es/m/[email protected] Discussion: https://postgr.es/m/[email protected] Branch ------ master Details ------- http://git.postgresql.org/pg/commitdiff/ca5f88502096a39f831da74eb02ec9eb3310616d Modified Files -------------- src/backend/utils/adt/selfuncs.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) -- Sent via pgsql-committers mailing list ([email protected]) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-committers
