Free memory after building each statistics object

tvondra · tvondra · commit 83772cc78e03 · 2021-09-23T18:05:10.000+02:00
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
diff --git a/src/backend/statistics/extended_stats.c b/src/backend/statistics/extended_stats.c
@@ -125,14 +125,16 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
 	if (!natts)
 		return;
 
+	/* the list of stats has to be allocated outside the memory context */
+	pg_stext = table_open(StatisticExtRelationId, RowExclusiveLock);
+	statslist = 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);
-	statslist = fetch_statentries_for_relation(pg_stext, RelationGetRelid(onerel));
-
 	/* report this phase */
 	if (statslist != NIL)
 	{
@@ -234,14 +236,16 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
 		pgstat_progress_update_param(PROGRESS_ANALYZE_EXT_STATS_COMPUTED,
 									 ++ext_cnt);
 
-		/* free the build data (allocated as a single chunk) */
-		pfree(data);
+		/* free the data used for building this statistics object */
+		MemoryContextReset(cxt);
 	}
 
-	table_close(pg_stext, RowExclusiveLock);
-
 	MemoryContextSwitchTo(oldcxt);
 	MemoryContextDelete(cxt);
+
+	list_free(statslist);
+
+	table_close(pg_stext, RowExclusiveLock);
 }
 
 /*
