From 7cee08efee31fe315b9b195405b3986c6fc99c5f Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 31 Aug 2007 23:35:30 +0000 Subject: [PATCH] Apply a band-aid fix for the problem that 8.2 and up completely misestimate the number of rows likely to be produced by a query such as SELECT * FROM t1 LEFT JOIN t2 USING (key) WHERE t2.key IS NULL; What this is doing is selecting for t1 rows with no match in t2, and thus it may produce a significant number of rows even if the t2.key table column contains no nulls at all. 8.2 thinks the table column's null fraction is relevant and thus may estimate no rows out, which results in terrible plans if there are more joins above this one. A proper fix for this will involve passing much more information about the context of a clause to the selectivity estimator functions than we ever have. There's no time left to write such a patch for 8.3, and it wouldn't be back-patchable into 8.2 anyway. Instead, put in an ad-hoc test to defeat the normal table-stats-based estimation when an IS NULL test is evaluated at an outer join, and just use a constant estimate instead --- I went with 0.5 for lack of a better idea. This won't catch every case but it will catch the typical ways of writing such queries, and it seems unlikely to make things worse for other queries. --- src/backend/optimizer/path/clausesel.c | 9 ++++++--- src/backend/utils/adt/selfuncs.c | 17 +++++++++++++++-- src/include/utils/selfuncs.h | 4 ++-- 3 files changed, 23 insertions(+), 7 deletions(-) diff --git a/src/backend/optimizer/path/clausesel.c b/src/backend/optimizer/path/clausesel.c index f51bac9b0e..463408f26f 100644 --- a/src/backend/optimizer/path/clausesel.c +++ b/src/backend/optimizer/path/clausesel.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.82 2006/10/04 00:29:53 momjian Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.82.2.1 2007/08/31 23:35:29 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -218,7 +218,9 @@ clauselist_selectivity(PlannerInfo *root, s2 = rqlist->hibound + rqlist->lobound - 1.0; /* Adjust for double-exclusion of NULLs */ - s2 += nulltestsel(root, IS_NULL, rqlist->var, varRelid); + /* HACK: disable nulltestsel's special outer-join logic */ + s2 += nulltestsel(root, IS_NULL, rqlist->var, + varRelid, JOIN_INNER); /* * A zero or slightly negative s2 should be converted into a @@ -701,7 +703,8 @@ clause_selectivity(PlannerInfo *root, s1 = nulltestsel(root, ((NullTest *) clause)->nulltesttype, (Node *) ((NullTest *) clause)->arg, - varRelid); + varRelid, + jointype); } else if (IsA(clause, BooleanTest)) { diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 0fcfdcbf32..56248a93b6 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -15,7 +15,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.214.2.5 2007/05/05 17:05:55 mha Exp $ + * $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.214.2.6 2007/08/31 23:35:29 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -1386,11 +1386,24 @@ booltestsel(PlannerInfo *root, BoolTestType booltesttype, Node *arg, */ Selectivity nulltestsel(PlannerInfo *root, NullTestType nulltesttype, - Node *arg, int varRelid) + Node *arg, int varRelid, JoinType jointype) { VariableStatData vardata; double selec; + /* + * Special hack: an IS NULL test being applied at an outer join should not + * be taken at face value, since it's very likely being used to select the + * outer-side rows that don't have a match, and thus its selectivity has + * nothing whatever to do with the statistics of the original table + * column. We do not have nearly enough context here to determine its + * true selectivity, so for the moment punt and guess at 0.5. Eventually + * the planner should be made to provide enough info about the clause's + * context to let us do better. + */ + if (IS_OUTER_JOIN(jointype) && nulltesttype == IS_NULL) + return (Selectivity) 0.5; + examine_variable(root, arg, varRelid, &vardata); if (HeapTupleIsValid(vardata.statsTuple)) diff --git a/src/include/utils/selfuncs.h b/src/include/utils/selfuncs.h index 69a22b37b5..7976e67b2e 100644 --- a/src/include/utils/selfuncs.h +++ b/src/include/utils/selfuncs.h @@ -8,7 +8,7 @@ * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/utils/selfuncs.h,v 1.36 2006/10/04 00:30:11 momjian Exp $ + * $PostgreSQL: pgsql/src/include/utils/selfuncs.h,v 1.36.2.1 2007/08/31 23:35:30 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -149,7 +149,7 @@ extern Datum icnlikejoinsel(PG_FUNCTION_ARGS); extern Selectivity booltestsel(PlannerInfo *root, BoolTestType booltesttype, Node *arg, int varRelid, JoinType jointype); extern Selectivity nulltestsel(PlannerInfo *root, NullTestType nulltesttype, - Node *arg, int varRelid); + Node *arg, int varRelid, JoinType jointype); extern Selectivity scalararraysel(PlannerInfo *root, ScalarArrayOpExpr *clause, bool is_join_clause,