Transformations: skip merge when not enough dataframes (#36407)

packages/grafana-data/src/transformations/transformers/merge.test.ts

@@ -16,6 +16,20 @@ describe('Merge multiple to single', () => {
     mockTransformationsRegistry([mergeTransformer]);
   });
 
+  it('skip combine one serie', async () => {
+    const seriesA = toDataFrame({
+      name: 'A',
+      fields: [
+        { name: 'Time', type: FieldType.time, values: [1000] },
+        { name: 'Temp', type: FieldType.number, values: [1] },
+      ],
+    });
+    await expect(transformDataFrame([cfg], [seriesA])).toEmitValuesWith((received) => {
+      const result = received[0];
+      expect(seriesA).toBe(result[0]);
+    });
+  });
+
   it('combine two series into one', async () => {
     const seriesA = toDataFrame({
       name: 'A',

packages/grafana-data/src/transformations/transformers/merge.ts

@@ -22,7 +22,7 @@ export const mergeTransformer: DataTransformerInfo<MergeTransformerOptions> = {
   operator: (options) => (source) =>
     source.pipe(
       map((dataFrames) => {
-        if (!Array.isArray(dataFrames) || dataFrames.length === 0) {
+        if (!Array.isArray(dataFrames) || dataFrames.length <= 1) {
           return dataFrames;
         }