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

diff --git a/src/backend/statistics/extended_stats.c b/src/backend/statistics/extended_stats.c
index 8951543b7eb4235c50e1c6700072b9134065ecb2..e0aece6b51d419ab47d9526c4d682446daa14193 100644 (file)
--- a/src/backend/statistics/extended_stats.c
+++ b/src/backend/statistics/extended_stats.c
@@ -90,14 +90,15 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
        MemoryContext cxt;
        MemoryContext oldcxt;
 
+       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));
-
        foreach(lc, stats)
        {
                StatExtEntry *stat = (StatExtEntry *) lfirst(lc);
@@ -148,12 +149,17 @@ BuildRelationExtStatistics(Relation onerel, double totalrows,
 
                /* store the statistics in the catalog */
                statext_store(stat->statOid, ndistinct, dependencies, mcv, stats);
-       }
 
-       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);
 }
 
 /*