Add in logic to send root datasource as override if needed (#56904)

public/app/core/utils/explore.ts

@@ -268,13 +268,13 @@ export async function generateEmptyQuery(
   let defaultQuery: Partial<DataQuery> | undefined;
 
   // datasource override is if we have switched datasources with no carry-over - we want to create a new query with a datasource we define
+  // it's also used if there's a root datasource and there were no previous queries
   if (dataSourceOverride) {
     datasourceRef = dataSourceOverride;
   } else if (queries.length > 0 && queries[queries.length - 1].datasource) {
     // otherwise use last queries' datasource
     datasourceRef = queries[queries.length - 1].datasource;
   } else {
-    // if neither exists, use the default datasource
     datasourceInstance = await getDataSourceSrv().get();
     defaultQuery = datasourceInstance.getDefaultQuery?.(CoreApp.Explore);
     datasourceRef = datasourceInstance.getRef();

public/app/features/explore/state/query.ts

@@ -222,7 +222,18 @@ export const clearCacheAction = createAction<ClearCachePayload>('explore/clearCa
 export function addQueryRow(exploreId: ExploreId, index: number): ThunkResult<void> {
   return async (dispatch, getState) => {
     const queries = getState().explore[exploreId]!.queries;
-    const query = await generateEmptyQuery(queries, index);
+    let datasourceOverride = undefined;
+
+    // if this is the first query being added, check for a root datasource
+    // if it's not mixed, send it as an override. generateEmptyQuery doesn't have access to state
+    if (queries.length === 0) {
+      const rootDatasource = getState().explore[exploreId]!.datasourceInstance;
+      if (!config.featureToggles.exploreMixedDatasource || !rootDatasource?.meta.mixed) {
+        datasourceOverride = rootDatasource;
+      }
+    }
+
+    const query = await generateEmptyQuery(queries, index, datasourceOverride?.getRef());
 
     dispatch(addQueryRowAction({ exploreId, index, query }));
   };