optimize regex matching for empty label values in posting match (#14075)

Also update tests.

Signed-off-by: Ben Ye <benye@amazon.com>

tsdb/querier.go

@@ -357,8 +357,8 @@ func inversePostingsForMatcher(ctx context.Context, ix IndexReader, m *labels.Ma
 	}
 
 	res := vals[:0]
-	// If the inverse match is ="", we just want all the values.
+	// If the match before inversion was !="" or !~"", we just want all the values.
-	if m.Type == labels.MatchEqual && m.Value == "" {
+	if m.Value == "" && (m.Type == labels.MatchRegexp || m.Type == labels.MatchEqual) {
 		res = vals
 	} else {
 		count := 1

tsdb/querier_test.go

@@ -2808,6 +2808,13 @@ func TestPostingsForMatchers(t *testing.T) {
 			},
 		},
 		// Not regex.
+		{
+			matchers: []*labels.Matcher{labels.MustNewMatcher(labels.MatchNotRegexp, "i", "")},
+			exp: []labels.Labels{
+				labels.FromStrings("n", "1", "i", "a"),
+				labels.FromStrings("n", "1", "i", "b"),
+			},
+		},
 		{
 			matchers: []*labels.Matcher{labels.MustNewMatcher(labels.MatchNotRegexp, "n", "^1$")},
 			exp: []labels.Labels{