From 0ae8b300388c2a3eaf90e6e6f13d6be1f4d4ac2d Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Tue, 12 Apr 2011 01:59:51 -0400 Subject: [PATCH] Be more wary of missing statistics in eqjoinsel_semi(). In particular, if we don't have real ndistinct estimates for both sides, fall back to assuming that half of the left-hand rows have join partners. This is what was done in 8.2 and 8.3 (cf nulltestsel() in those versions). It's pretty stupid but it won't lead us to think that an antijoin produces no rows out, as seen in recent example from Uwe Schroeder. --- src/backend/utils/adt/selfuncs.c | 49 +++++++++++++++++++++----------- 1 file changed, 32 insertions(+), 17 deletions(-) diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 38a8a89ad3..3333b7a808 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -2261,7 +2261,9 @@ eqjoinsel_semi(Oid operator, bool *hasmatch1; bool *hasmatch2; double nullfrac1 = stats1->stanullfrac; - double matchfreq1; + double matchfreq1, + uncertainfrac, + uncertain; int i, nmatches; @@ -2314,18 +2316,26 @@ eqjoinsel_semi(Oid operator, * the uncertain rows that a fraction nd2/nd1 have join partners. We * can discount the known-matched MCVs from the distinct-values counts * before doing the division. + * + * Crude as the above is, it's completely useless if we don't have + * reliable ndistinct values for both sides. Hence, if either nd1 + * or nd2 is default, punt and assume half of the uncertain rows + * have join partners. */ - nd1 -= nmatches; - nd2 -= nmatches; - if (nd1 <= nd2 || nd2 <= 0) - selec = Max(matchfreq1, 1.0 - nullfrac1); - else + if (nd1 != DEFAULT_NUM_DISTINCT && nd2 != DEFAULT_NUM_DISTINCT) { - double uncertain = 1.0 - matchfreq1 - nullfrac1; - - CLAMP_PROBABILITY(uncertain); - selec = matchfreq1 + (nd2 / nd1) * uncertain; + nd1 -= nmatches; + nd2 -= nmatches; + if (nd1 <= nd2 || nd2 <= 0) + uncertainfrac = 1.0; + else + uncertainfrac = nd2 / nd1; } + else + uncertainfrac = 0.5; + uncertain = 1.0 - matchfreq1 - nullfrac1; + CLAMP_PROBABILITY(uncertain); + selec = matchfreq1 + uncertainfrac * uncertain; } else { @@ -2335,15 +2345,20 @@ eqjoinsel_semi(Oid operator, */ double nullfrac1 = stats1 ? stats1->stanullfrac : 0.0; - if (vardata1->rel) - nd1 = Min(nd1, vardata1->rel->rows); - if (vardata2->rel) - nd2 = Min(nd2, vardata2->rel->rows); + if (nd1 != DEFAULT_NUM_DISTINCT && nd2 != DEFAULT_NUM_DISTINCT) + { + if (vardata1->rel) + nd1 = Min(nd1, vardata1->rel->rows); + if (vardata2->rel) + nd2 = Min(nd2, vardata2->rel->rows); - if (nd1 <= nd2 || nd2 <= 0) - selec = 1.0 - nullfrac1; + if (nd1 <= nd2 || nd2 <= 0) + selec = 1.0 - nullfrac1; + else + selec = (nd2 / nd1) * (1.0 - nullfrac1); + } else - selec = (nd2 / nd1) * (1.0 - nullfrac1); + selec = 0.5 * (1.0 - nullfrac1); } if (have_mcvs1) -- 2.50.0