From 70dac855e18ce19aad4d0c9b98028da1a6d9e470 Mon Sep 17 00:00:00 2001 From: Evdokimov Ilia Date: Fri, 9 Oct 2026 00:02:22 +0500 Subject: [PATCH v1] Release memory allocated by ndistinct_for_combination Building ndistinct extended statistics calls ndistinct_for_combination() once for every combination of columns. Each call allocates arrays for the whole sample (SortItems, Datums and null flags), plus the MultiSortSupport, and never frees them, so the memory accumulates until the whole statistics object is built. The sample has 300 * target rows, so with large statistics targets this can add up to gigabytes; Fixed by calling ndistinct_for_combination() in a dedicated memory context and resetting it after each combination, as is already done for dependency_degree() in statext_dependencies_build(). We only need the estimated number of distinct values, so we don't need to copy anything out of the context. --- src/backend/statistics/mvdistinct.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/src/backend/statistics/mvdistinct.c b/src/backend/statistics/mvdistinct.c index 601f108a2b4..0add6455f6a 100644 --- a/src/backend/statistics/mvdistinct.c +++ b/src/backend/statistics/mvdistinct.c @@ -28,6 +28,7 @@ #include "catalog/pg_statistic_ext.h" #include "catalog/pg_statistic_ext_data.h" #include "statistics/extended_stats_internal.h" +#include "utils/memutils.h" #include "utils/syscache.h" #include "utils/typcache.h" #include "varatt.h" @@ -89,6 +90,7 @@ statext_ndistinct_build(double totalrows, StatsBuildData *data) uint32 itemcnt; int numattrs = data->nattnums; int numcombs = num_combinations(numattrs); + MemoryContext cxt; result = palloc(offsetof(MVNDistinct, items) + numcombs * sizeof(MVNDistinctItem)); @@ -96,6 +98,10 @@ statext_ndistinct_build(double totalrows, StatsBuildData *data) result->type = STATS_NDISTINCT_TYPE_BASIC; result->nitems = numcombs; + cxt = AllocSetContextCreate(CurrentMemoryContext, + "ndistinct_for_combination cxt", + ALLOCSET_DEFAULT_SIZES); + itemcnt = 0; for (k = 2; k <= numattrs; k++) { @@ -109,6 +115,7 @@ statext_ndistinct_build(double totalrows, StatsBuildData *data) { MVNDistinctItem *item = &result->items[itemcnt]; int j; + MemoryContext oldcxt; item->attributes = palloc_array(AttrNumber, k); item->nattributes = k; @@ -121,8 +128,12 @@ statext_ndistinct_build(double totalrows, StatsBuildData *data) Assert(AttributeNumberIsValid(item->attributes[j])); } + /* release memory used by ndistinct calculation */ + oldcxt = MemoryContextSwitchTo(cxt); item->ndistinct = ndistinct_for_combination(totalrows, data, k, combination); + MemoryContextSwitchTo(oldcxt); + MemoryContextReset(cxt); itemcnt++; Assert(itemcnt <= result->nitems); @@ -131,6 +142,8 @@ statext_ndistinct_build(double totalrows, StatsBuildData *data) generator_free(generator); } + MemoryContextDelete(cxt); + /* must consume exactly the whole output array */ Assert(itemcnt == result->nitems); -- 2.53.0