From ca5f88502096a39f831da74eb02ec9eb3310616d Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Tue, 29 Nov 2016 18:00:49 -0500 Subject: [PATCH] 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/20161128182113.6527.58926@wrigleys.postgresql.org Discussion: https://postgr.es/m/31089.1480384713@sss.pgh.pa.us --- src/backend/utils/adt/selfuncs.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 56943f2a87..4973396b80 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -2511,10 +2511,24 @@ eqjoinsel_semi(Oid operator, * We can apply this clamping both with respect to the base relation from * which the join variable comes (if there is just one), and to the * immediate inner input relation of the current join. + * + * If we clamp, we can treat nd2 as being a non-default estimate; it's not + * great, maybe, but it didn't come out of nowhere either. This is most + * helpful when the inner relation is empty and consequently has no stats. */ if (vardata2->rel) - nd2 = Min(nd2, vardata2->rel->rows); - nd2 = Min(nd2, inner_rel->rows); + { + if (nd2 >= vardata2->rel->rows) + { + nd2 = vardata2->rel->rows; + isdefault2 = false; + } + } + if (nd2 >= inner_rel->rows) + { + nd2 = inner_rel->rows; + isdefault2 = false; + } if (HeapTupleIsValid(vardata1->statsTuple)) {