Only do regex lookups when there was no equality match.

For the label matching index-based preselection phase, don't do an OR
between equality and non-equality matchers. Execute only one of the two
(with equality matchers preferred when present).

Fixes https://github.com/prometheus/prometheus/issues/924

storage/local/storage.go

@@ -428,11 +428,11 @@ func (s *memorySeriesStorage) MetricsForLabelMatchers(matchers ...*metric.LabelM
 	}
 
 	var resFPs map[clientmodel.Fingerprint]struct{}
-	// If we cannot make a preselection based on equality matchers, expanding the other matchers to labels
-	// and intersecting their fingerprints is still likely to be the best choice.
 	if len(equals) > 0 {
 		resFPs = s.fingerprintsForLabelPairs(equals...)
-	}
+	} else {
+		// If we cannot make a preselection based on equality matchers, expanding the other matchers to labels
+		// and intersecting their fingerprints is still likely to be the best choice.
 		var remaining metric.LabelMatchers
 		for _, matcher := range filters {
 			// Equal matches are all empty values.
@@ -461,6 +461,7 @@ func (s *memorySeriesStorage) MetricsForLabelMatchers(matchers ...*metric.LabelM
 		}
 		// The intersected matchers no longer need to be compared against the actual metrics.
 		filters = remaining
+	}
 
 	result := make(map[clientmodel.Fingerprint]clientmodel.COWMetric, len(resFPs))
 	for fp := range resFPs {

storage/local/storage_test.go

@@ -70,7 +70,7 @@ func TestMatches(t *testing.T) {
 	}{
 		{
 			matchers: metric.LabelMatchers{newMatcher(metric.Equal, "label1", "x")},
-			expected: fingerprints[:0],
+			expected: clientmodel.Fingerprints{},
 		},
 		{
 			matchers: metric.LabelMatchers{newMatcher(metric.Equal, "label1", "test_0")},
@@ -161,6 +161,20 @@ func TestMatches(t *testing.T) {
 			},
 			expected: append(append(clientmodel.Fingerprints{}, fingerprints[30:35]...), fingerprints[45:60]...),
 		},
+		{
+			matchers: metric.LabelMatchers{
+				newMatcher(metric.Equal, "label1", `nonexistent`),
+				newMatcher(metric.RegexMatch, "label2", `test`),
+			},
+			expected: clientmodel.Fingerprints{},
+		},
+		{
+			matchers: metric.LabelMatchers{
+				newMatcher(metric.Equal, "label1", `test_0`),
+				newMatcher(metric.RegexMatch, "label2", `nonexistent`),
+			},
+			expected: clientmodel.Fingerprints{},
+		},
 	}
 
 	for _, mt := range matcherTests {