Fix compareLabels, add test

querier.go

@@ -182,7 +182,7 @@ func compareLabels(a, b Labels) int {
 		}
 	}
 	// If all labels so far were in common, the set with fewer labels comes first.
-	return len(b) - len(a)
+	return len(a) - len(b)
 }
 
 func (s *shardSeriesSet) Series() Series {

querier_test.go

@@ -0,0 +1,46 @@
+package tsdb
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCompareLabels(t *testing.T) {
+	cases := []struct {
+		a, b []Label
+		res  int
+	}{
+		{
+			a:   []Label{},
+			b:   []Label{},
+			res: 0,
+		},
+		{
+			a:   []Label{{"a", ""}},
+			b:   []Label{{"a", ""}, {"b", ""}},
+			res: -1,
+		},
+		{
+			a:   []Label{{"a", ""}},
+			b:   []Label{{"a", ""}},
+			res: 0,
+		},
+		{
+			a:   []Label{{"aa", ""}, {"aa", ""}},
+			b:   []Label{{"aa", ""}, {"ab", ""}},
+			res: -1,
+		},
+		{
+			a:   []Label{{"aa", ""}, {"abb", ""}},
+			b:   []Label{{"aa", ""}, {"ab", ""}},
+			res: 1,
+		},
+	}
+	for _, c := range cases {
+		// Use constructor to ensure sortedness.
+		a, b := NewLabels(c.a...), NewLabels(c.b...)
+
+		require.Equal(t, c.res, compareLabels(a, b))
+	}
+}