Add shortcuts for empty postings

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/head_read.go

@@ -113,7 +113,9 @@ func (h *headIndexReader) Postings(name string, values ...string) (index.Posting
 	default:
 		res := make([]index.Postings, 0, len(values))
 		for _, value := range values {
-			res = append(res, h.head.postings.Get(name, value))
+			if p := h.head.postings.Get(name, value); !index.IsEmptyPostings(p) {
+				res = append(res, p)
+			}
 		}
 		return index.Merge(res...), nil
 	}

tsdb/index/postings.go

@@ -428,6 +428,13 @@ func EmptyPostings() Postings {
 	return emptyPostings
 }
 
+// IsEmptyPostings returns true if the postings are an empty postings list.
+// When this function returns false, it doesn't mean that the postings isn't empty
+// (it could be an empty intersection of two non-empty postings, for example).
+func IsEmptyPostings(p Postings) bool {
+	return p == emptyPostings
+}
+
 // ErrPostings returns new postings that immediately error.
 func ErrPostings(err error) Postings {
 	return errPostings{err}

tsdb/querier.go

@@ -215,6 +215,9 @@ func PostingsForMatchers(ix IndexPostingsReader, ms ...*labels.Matcher) (index.P
 				if err != nil {
 					return nil, err
 				}
+				if index.IsEmptyPostings(it) {
+					return index.EmptyPostings(), nil
+				}
 				its = append(its, it)
 			} else { // l="a"
 				// Non-Not matcher, use normal postingsForMatcher.
@@ -222,6 +225,9 @@ func PostingsForMatchers(ix IndexPostingsReader, ms ...*labels.Matcher) (index.P
 				if err != nil {
 					return nil, err
 				}
+				if index.IsEmptyPostings(it) {
+					return index.EmptyPostings(), nil
+				}
 				its = append(its, it)
 			}
 		} else { // l=""