scorch zap optimize when zero hits

Instead of allocating brand-new empty postingsList/Iterator instances,
reuse some empty singletons.

index/scorch/segment/zap/dict.go

@@ -39,6 +39,9 @@ func (d *Dictionary) PostingsList(term string, except *roaring.Bitmap) (segment.
 
 func (d *Dictionary) postingsList(term []byte, except *roaring.Bitmap, rv *PostingsList) (*PostingsList, error) {
 	if d.fst == nil {
+		if rv == nil || rv == emptyPostingsList {
+			return emptyPostingsList, nil
+		}
 		return d.postingsListInit(rv, except), nil
 	}
 
@@ -47,6 +50,9 @@ func (d *Dictionary) postingsList(term []byte, except *roaring.Bitmap, rv *Posti
 		return nil, fmt.Errorf("vellum err: %v", err)
 	}
 	if !exists {
+		if rv == nil || rv == emptyPostingsList {
+			return emptyPostingsList, nil
+		}
 		return d.postingsListInit(rv, except), nil
 	}
 
@@ -65,7 +71,7 @@ func (d *Dictionary) postingsListFromOffset(postingsOffset uint64, except *roari
 }
 
 func (d *Dictionary) postingsListInit(rv *PostingsList, except *roaring.Bitmap) *PostingsList {
-	if rv == nil {
+	if rv == nil || rv == emptyPostingsList {
 		rv = &PostingsList{}
 	} else {
 		postings := rv.postings

index/scorch/segment/zap/posting.go

@@ -109,6 +109,9 @@ type PostingsList struct {
 	normBits1Hit uint64
 }
 
+// represents an immutable, empty postings list
+var emptyPostingsList = &PostingsList{}
+
 func (p *PostingsList) Size() int {
 	sizeInBytes := reflectStaticSizePostingsList + size.SizeOfPtr
 
@@ -132,6 +135,10 @@ func (p *PostingsList) OrInto(receiver *roaring.Bitmap) {
 
 // Iterator returns an iterator for this postings list
 func (p *PostingsList) Iterator(includeFreq, includeNorm, includeLocs bool) segment.PostingsIterator {
+	if p.normBits1Hit == 0 && p.postings == nil {
+		return emptyPostingsIterator
+	}
+
 	return p.iterator(includeFreq, includeNorm, includeLocs, nil)
 }
 
@@ -341,6 +348,8 @@ type PostingsIterator struct {
 	includeLocs     bool
 }
 
+var emptyPostingsIterator = &PostingsIterator{}
+
 func (i *PostingsIterator) Size() int {
 	sizeInBytes := reflectStaticSizePostingsIterator + size.SizeOfPtr +
 		len(i.currChunkFreqNorm) +
@@ -589,6 +598,10 @@ func (i *PostingsIterator) nextBytes() (
 }
 
 func (i *PostingsIterator) Advance(docNumber uint64) (segment.Posting, error) {
+	if i.postings == nil {
+		return nil, nil
+	}
+
 	// check if we are already there
 	if i.next.Number() == docNumber {
 		return &i.next, nil

index/scorch/segment/zap/segment.go

@@ -373,7 +373,7 @@ func (s *SegmentBase) DocNumbers(ids []string) (*roaring.Bitmap, error) {
 			return nil, err
 		}
 
-		var postingsList *PostingsList
+		postingsList := emptyPostingsList
 		for _, id := range ids {
 			postingsList, err = idDict.postingsList([]byte(id), nil, postingsList)
 			if err != nil {