Add fast-path for equality matching

db_windows.go

@@ -7,20 +7,20 @@ import (
 )
 
 func mmap(f *os.File, sz int) ([]byte, error) {
-	low, high := uint32(size), uint32(size>>32)
+	low, high := uint32(sz), uint32(sz>>32)
 
 	h, errno := syscall.CreateFileMapping(syscall.Handle(f.Fd()), nil, syscall.PAGE_READONLY, low, high, nil)
 	if h == 0 {
-		return os.NewSyscallError("CreateFileMapping", errno)
+		return nil, os.NewSyscallError("CreateFileMapping", errno)
 	}
 
 	addr, errno := syscall.MapViewOfFile(h, syscall.FILE_MAP_READ, 0, 0, uintptr(sz))
 	if addr == 0 {
-		return os.NewSyscallError("MapViewOfFile", errno)
+		return nil, os.NewSyscallError("MapViewOfFile", errno)
 	}
 
 	if err := syscall.CloseHandle(syscall.Handle(h)); err != nil {
-		return os.NewSyscallError("CloseHandle", err)
+		return nil, os.NewSyscallError("CloseHandle", err)
 	}
 
 	return (*[1 << 30]byte)(unsafe.Pointer(addr))[:sz], nil

index.go

@@ -688,7 +688,7 @@ func (r *indexReader) Postings(name, value string) (Postings, error) {
 
 	off, ok := r.postings[key]
 	if !ok {
-		return nil, ErrNotFound
+		return emptyPostings, nil
 	}
 
 	flag, b, err := r.section(off)

labels/selector.go

@@ -34,6 +34,9 @@ func (m *EqualMatcher) Name() string { return m.name }
 // Matches implements Matcher interface.
 func (m *EqualMatcher) Matches(v string) bool { return v == m.value }
 
+// Value returns the matched value.
+func (m *EqualMatcher) Value() string { return m.value }
+
 // NewEqualMatcher returns a new matcher matching an exact label value.
 func NewEqualMatcher(name, value string) Matcher {
 	return &EqualMatcher{name: name, value: value}

querier.go

@@ -157,6 +157,15 @@ func (q *blockQuerier) Select(ms ...labels.Matcher) SeriesSet {
 }
 
 func (q *blockQuerier) selectSingle(m labels.Matcher) Postings {
+	// Fast-path for equal matching.
+	if em, ok := m.(*labels.EqualMatcher); ok {
+		it, err := q.index.Postings(em.Name(), em.Value())
+		if err != nil {
+			return errPostings{err: err}
+		}
+		return it
+	}
+
 	tpls, err := q.index.LabelValues(m.Name())
 	if err != nil {
 		return errPostings{err: err}
@@ -174,7 +183,6 @@ func (q *blockQuerier) selectSingle(m labels.Matcher) Postings {
 			res = append(res, vals[0])
 		}
 	}
-
 	if len(res) == 0 {
 		return emptyPostings
 	}