SQL: Fixes issues with showing value column name prefix in legends (#35839)

* rename strategy

* Update pkg/tsdb/sqleng/sql_engine.go

Co-authored-by: Torkel Ödegaard <torkel@grafana.org>

* more strict constraints

* Fixed so that it works on multi series results

* only apply the logic when original query returns 3 fields

* removed part of comment

* Update mysql test

Co-authored-by: Torkel Ödegaard <torkel@grafana.org>
Co-authored-by: Torkel Ödegaard <torkel@grafana.com>

pkg/tsdb/mysql/mysql_test.go

@@ -732,8 +732,8 @@ func TestMySQL(t *testing.T) {
 			frames, _ := queryResult.Dataframes.Decoded()
 			require.Len(t, frames, 1)
 			require.Len(t, frames[0].Fields, 3)
-			require.Equal(t, data.Labels{"metric": "Metric A - value one"}, frames[0].Fields[1].Labels)
-			require.Equal(t, data.Labels{"metric": "Metric B - value one"}, frames[0].Fields[2].Labels)
+			require.Equal(t, "Metric A - value one", frames[0].Fields[1].Name)
+			require.Equal(t, "Metric B - value one", frames[0].Fields[2].Name)
 		})
 
 		t.Run("When doing a metric query with metric column and multiple value columns", func(t *testing.T) {

pkg/tsdb/postgres/postgres_test.go

@@ -795,8 +795,8 @@ func TestPostgres(t *testing.T) {
 			frames, _ := queryResult.Dataframes.Decoded()
 			require.Equal(t, 1, len(frames))
 			require.Equal(t, 3, len(frames[0].Fields))
-			require.Equal(t, data.Labels{"metric": "Metric A - value one"}, frames[0].Fields[1].Labels)
-			require.Equal(t, data.Labels{"metric": "Metric B - value one"}, frames[0].Fields[2].Labels)
+			require.Equal(t, "Metric A - value one", frames[0].Fields[1].Name)
+			require.Equal(t, "Metric B - value one", frames[0].Fields[2].Name)
 		})
 
 		t.Run("When doing a metric query with metric column and multiple value columns", func(t *testing.T) {

pkg/tsdb/sqleng/sql_engine.go

@@ -298,11 +298,27 @@ func (e *dataPlugin) executeQuery(query plugins.DataSubQuery, wg *sync.WaitGroup
 		tsSchema := frame.TimeSeriesSchema()
 		if tsSchema.Type == data.TimeSeriesTypeLong {
 			var err error
+			originalData := frame
 			frame, err = data.LongToWide(frame, qm.FillMissing)
 			if err != nil {
 				errAppendDebug("failed to convert long to wide series when converting from dataframe", err, interpolatedQuery)
 				return
 			}
+
+			// Before 8x, a special metric column was used to name time series. The LongToWide transforms that into a metric label on the value field.
+			// But that makes series name have both the value column name AND the metric name. So here we are removing the metric label here and moving it to the
+			// field name to get the same naming for the series as pre v8
+			if len(originalData.Fields) == 3 {
+				for _, field := range frame.Fields {
+					if len(field.Labels) == 1 { // 7x only supported one label
+						name, ok := field.Labels["metric"]
+						if ok {
+							field.Name = name
+							field.Labels = nil
+						}
+					}
+				}
+			}
 		}
 		if qm.FillMissing != nil {
 			var err error