Skip truncating ON COMMIT DELETE ROWS temp tables, if the transaction hasn't

touched any temporary tables.

We could try harder, and keep track of whether we've inserted to any temp
tables, rather than accessed them, and which temp tables have been inserted
to. But this is dead simple, and already covers many interesting scenarios.

Author: hlinnaka

src/backend/commands/tablecmds.c

@@ -10124,7 +10124,13 @@ PreCommit_on_commit_actions(void)
 				/* Do nothing (there shouldn't be such entries, actually) */
 				break;
 			case ONCOMMIT_DELETE_ROWS:
-				oids_to_truncate = lappend_oid(oids_to_truncate, oc->relid);
+				/*
+				 * If this transaction hasn't accessed any temporary
+				 * relations, we can skip truncating ON COMMIT DELETE ROWS
+				 * tables, as they must still be empty.
+				 */
+				if (MyXactAccessedTempRel)
+					oids_to_truncate = lappend_oid(oids_to_truncate, oc->relid);
 				break;
 			case ONCOMMIT_DROP:
 				{