Throw an error if placements cannot be found in router executor

src/backend/distributed/executor/multi_router_executor.c

@@ -628,6 +628,13 @@ ExecuteSingleSelectTask(CitusScanState *scanState, Task *task)
 			placementAccessList = list_make1(placementAccess);
 		}
 
+		if (placementAccessList == NIL)
+		{
+			ereport(ERROR, (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
+							errmsg("a placement was moved after the SELECT was "
+								   "planned")));
+		}
+
 		connection = GetPlacementListConnection(connectionFlags, placementAccessList,
 												NULL);