Skip allocating hash table in EXPLAIN-only mode.

hlinnaka · hlinnaka · commit 57b5d8484c8a · 2020-11-20T14:49:25.000+02:00
This is a backpatch of commit 2cccb62, backpatched due to popular demand. Backpatch to all supported versions. Author: Alexey Bashtanov Discussion: https://www.postgresql.org/message-id/36823f65-050d-ae24-aa4d-a37726998240%40imap.cc
diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
@@ -2454,7 +2454,11 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 		aggstate->hash_pergroup = pergroups;
 
 		find_hash_columns(aggstate);
-		build_hash_table(aggstate);
+
+		/* Skip massive memory allocation if we are just doing EXPLAIN */
+		if (!(eflags & EXEC_FLAG_EXPLAIN_ONLY))
+			build_hash_table(aggstate);
+
 		aggstate->table_filled = false;
 	}
 
