From 66b16792e8e16dad33adb5614a0936cc41002f4c Mon Sep 17 00:00:00 2001 From: David Geier Date: Fri, 18 Nov 2022 09:35:08 +0100 Subject: [PATCH] Don't read MCV stats needlessly in join selectivity estimation Join selectivity estimation only uses MCV stats if they're present on both join attributes. Therefore, if MCV stats are not available on one of the two columns, skip reading MCV stats altogether. Frequently encountered situations in which MCV stats not available is unique columns or columns for which all values have roughly the same frequency and therefore ANALYZE decided to not store them. --- src/backend/utils/adt/selfuncs.c | 15 +++++++++++++-- src/backend/utils/cache/lsyscache.c | 4 ++++ 2 files changed, 17 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index d597b7e81f..5baab4e3ac 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -2263,6 +2263,7 @@ eqjoinsel(PG_FUNCTION_ARGS) bool have_mcvs2 = false; bool join_is_reversed; RelOptInfo *inner_rel; + bool get_mcv_stats; get_join_variables(root, args, sjinfo, &vardata1, &vardata2, &join_is_reversed); @@ -2275,11 +2276,21 @@ eqjoinsel(PG_FUNCTION_ARGS) memset(&sslot1, 0, sizeof(sslot1)); memset(&sslot2, 0, sizeof(sslot2)); + /* + * There is no use in fetching one side's MCVs if we lack MCVs for the + * other side, so do a quick check to verify that both stats exist. + */ + get_mcv_stats = (HeapTupleIsValid(vardata1.statsTuple) && + HeapTupleIsValid(vardata2.statsTuple) && + get_attstatsslot(&sslot1, vardata1.statsTuple, STATISTIC_KIND_MCV, InvalidOid, 0) && + get_attstatsslot(&sslot2, vardata2.statsTuple, STATISTIC_KIND_MCV, InvalidOid, 0)); + + if (HeapTupleIsValid(vardata1.statsTuple)) { /* note we allow use of nullfrac regardless of security check */ stats1 = (Form_pg_statistic) GETSTRUCT(vardata1.statsTuple); - if (statistic_proc_security_check(&vardata1, opfuncoid)) + if (get_mcv_stats && statistic_proc_security_check(&vardata1, opfuncoid)) have_mcvs1 = get_attstatsslot(&sslot1, vardata1.statsTuple, STATISTIC_KIND_MCV, InvalidOid, ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS); @@ -2289,7 +2300,7 @@ eqjoinsel(PG_FUNCTION_ARGS) { /* note we allow use of nullfrac regardless of security check */ stats2 = (Form_pg_statistic) GETSTRUCT(vardata2.statsTuple); - if (statistic_proc_security_check(&vardata2, opfuncoid)) + if (get_mcv_stats && statistic_proc_security_check(&vardata2, opfuncoid)) have_mcvs2 = get_attstatsslot(&sslot2, vardata2.statsTuple, STATISTIC_KIND_MCV, InvalidOid, ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS); diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c index a16a63f495..191f68f7b4 100644 --- a/src/backend/utils/cache/lsyscache.c +++ b/src/backend/utils/cache/lsyscache.c @@ -3157,6 +3157,10 @@ get_attavgwidth(Oid relid, AttrNumber attnum) * reqkind: STAKIND code for desired statistics slot kind. * reqop: STAOP value wanted, or InvalidOid if don't care. * flags: bitmask of ATTSTATSSLOT_VALUES and/or ATTSTATSSLOT_NUMBERS. + * Passing 0 will only check if the requested slot type exists but not + * extract any content. This is useful in cases where MCV stats are only + * useful if they exists on all involved columns (e.g. during join + * selectivity computation). * * If a matching slot is found, true is returned, and *sslot is filled thus: * staop: receives the actual STAOP value. -- 2.34.1