fix where constraint handling

public/app/plugins/datasource/postgres/query_ctrl.ts

@@ -303,6 +303,7 @@ export class PostgresQueryCtrl extends QueryCtrl {
       }
     }
     this.whereAdd = this.uiSegmentSrv.newPlusButton();
+    this.queryModel.updatePersistedParts();
     this.panelCtrl.refresh();
   }