Free memory after building each statistics object

Until now, all extended statistics on a given relation were built in the
same memory context, without resetting. Some of the memory was released
explicitly, but not all of it - for example memory allocated while
detoasting values is hard to free. This is how it worked since extended
statistics were introduced in PostgreSQL 10, but adding support for
extended stats on expressions made the issue somewhat worse as it
increases the number of statistics to build.

Fixed by adding a memory context which gets reset after building each
statistics object (all the statistics kinds included in it). Resetting
it after building each statistics kind would be even better, but it
would require more invasive changes and copying of results, making it
harder to backpatch.

Backpatch to PostgreSQL 10, where extended statistics were introduced.

Author: Justin Pryzby
Reported-by: Justin Pryzby
Reviewed-by: Tomas Vondra
Backpatch-through: 10
Discussion: https://www.postgresql.org/message-id/20210915200928.GP831%40telsasoft.com

1 files changed, 11 insertions, 5 deletions

diff --git a/src/backend/statistics/extended_stats.c b/src/backend/statistics/extended_stats.c
index 0c80e55d107..4c19336b293 100644
--- a/src/backend/statistics/extended_stats.c
+++ b/src/backend/statistics/extended_stats.c
@@ -97,14 +97,15 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
 	MemoryContext oldcxt;
 	int64		ext_cnt;
 
+	pg_stext = table_open(StatisticExtRelationId, RowExclusiveLock);
+	stats = fetch_statentries_for_relation(pg_stext, RelationGetRelid(onerel));
+
+	/* memory context for building each statistics object */
 	cxt = AllocSetContextCreate(CurrentMemoryContext,
 								"BuildRelationExtStatistics",
 								ALLOCSET_DEFAULT_SIZES);
 	oldcxt = MemoryContextSwitchTo(cxt);
 
-	pg_stext = table_open(StatisticExtRelationId, RowExclusiveLock);
-	stats = fetch_statentries_for_relation(pg_stext, RelationGetRelid(onerel));
-
 	/* report this phase */
 	if (stats != NIL)
 	{
@@ -189,12 +190,17 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
 		/* for reporting progress */
 		pgstat_progress_update_param(PROGRESS_ANALYZE_EXT_STATS_COMPUTED,
 									 ++ext_cnt);
-	}
 
-	table_close(pg_stext, RowExclusiveLock);
+		/* free the data used for building this statistics object */
+		MemoryContextReset(cxt);
+	}
 
 	MemoryContextSwitchTo(oldcxt);
 	MemoryContextDelete(cxt);
+
+	list_free(stats);
+
+	table_close(pg_stext, RowExclusiveLock);
 }
 
 /*