Revert "[Data Explorer][Discover 2.0] Fix issues when change index pa…

…ttern (opensearch-project#4875) (opensearch-project#4885)"

This reverts commit 0102a32.
try revert:

src/plugins/discover/public/application/components/sidebar/lib/get_index_pattern_field_list.ts

@@ -36,5 +36,18 @@ export function getIndexPatternFieldList(
   fieldCounts?: Record<string, number>
 ) {
   if (!indexPattern || !fieldCounts) return [];
-  return [...indexPattern.fields.getAll()];
+
+  const fieldNamesInDocs = Object.keys(fieldCounts);
+  const fieldNamesInIndexPattern = indexPattern.fields.getAll().map((fld) => fld.name);
+  const unknownTypes: IndexPatternField[] = [];
+
+  difference(fieldNamesInDocs, fieldNamesInIndexPattern).forEach((unknownFieldName) => {
+    unknownTypes.push({
+      displayName: String(unknownFieldName),
+      name: String(unknownFieldName),
+      type: 'unknown',
+    } as IndexPatternField);
+  });
+
+  return [...indexPattern.fields.getAll(), ...unknownTypes];
 }

src/plugins/discover/public/application/view_components/canvas/index.tsx

@@ -25,17 +25,6 @@ import './discover_canvas.scss';
 // eslint-disable-next-line import/no-default-export
 export default function DiscoverCanvas({ setHeaderActionMenu, history }: ViewProps) {
   const { data$, refetch$, indexPattern } = useDiscoverContext();
-  const {
-    services: { uiSettings },
-  } = useOpenSearchDashboards<DiscoverViewServices>();
-  const { columns } = useSelector((state) => state.discover);
-  const filteredColumns = filterColumns(
-    columns,
-    indexPattern,
-    uiSettings.get(DEFAULT_COLUMNS_SETTING)
-  );
-  const dispatch = useDispatch();
-  const prevIndexPattern = useRef(indexPattern);
 
   const [fetchState, setFetchState] = useState<SearchData>({
     status: data$.getValue().status,
@@ -80,13 +69,6 @@ export default function DiscoverCanvas({ setHeaderActionMenu, history }: ViewPro
     };
   }, [data$, fetchState]);
 
-  useEffect(() => {
-    if (indexPattern !== prevIndexPattern.current) {
-      dispatch(setColumns({ columns: filteredColumns }));
-      prevIndexPattern.current = indexPattern;
-    }
-  }, [dispatch, filteredColumns, indexPattern]);
-
   const timeField = indexPattern?.timeFieldName ? indexPattern.timeFieldName : undefined;
 
   return (