scorch zap optimize via postingsList reuse

pprof graphs were showing many postingsList allocations during
merging, so this change optimizes by reusing postingList memory in the
merging loops.

index/scorch/segment/zap/dict.go

@@ -34,15 +34,18 @@ type Dictionary struct {
 
 // PostingsList returns the postings list for the specified term
 func (d *Dictionary) PostingsList(term string, except *roaring.Bitmap) (segment.PostingsList, error) {
-	return d.postingsList([]byte(term), except)
+	return d.postingsList([]byte(term), except, nil)
 }
 
-func (d *Dictionary) postingsList(term []byte, except *roaring.Bitmap) (*PostingsList, error) {
-	rv := &PostingsList{
-		sb:     d.sb,
-		term:   term,
-		except: except,
+func (d *Dictionary) postingsList(term []byte, except *roaring.Bitmap, rv *PostingsList) (*PostingsList, error) {
+	if rv == nil {
+		rv = &PostingsList{}
+	} else {
+		*rv = PostingsList{} // clear the struct
 	}
+	rv.sb = d.sb
+	rv.term = term
+	rv.except = except
 
 	if d.fst != nil {
 		postingsOffset, exists, err := d.fst.Get(term)

index/scorch/segment/zap/merge.go

@@ -155,6 +155,8 @@ func persistMergedRest(segments []*Segment, drops []*roaring.Bitmap,
 	var bufMaxVarintLen64 []byte = make([]byte, binary.MaxVarintLen64)
 	var bufLoc []uint64
 
+	var postings *PostingsList
+
 	rv := make([]uint64, len(fieldsInv))
 	fieldDvLocs := make([]uint64, len(fieldsInv))
 	fieldDvLocsOffset := uint64(fieldNotUninverted)
@@ -231,7 +233,8 @@ func persistMergedRest(segments []*Segment, drops []*roaring.Bitmap,
 				if dict == nil {
 					continue
 				}
-				postings, err2 := dict.postingsList(term, drops[dictI])
+				var err2 error
+				postings, err2 = dict.postingsList(term, drops[dictI], postings)
 				if err2 != nil {
 					return nil, 0, err2
 				}

index/scorch/segment/zap/merge_test.go

@@ -310,8 +310,8 @@ func compareSegments(a, b *Segment) string {
 					continue
 				}
 
-				aplist, aerr := adict.(*Dictionary).postingsList([]byte(next.Term), nil)
-				bplist, berr := bdict.(*Dictionary).postingsList([]byte(next.Term), nil)
+				aplist, aerr := adict.(*Dictionary).postingsList([]byte(next.Term), nil, nil)
+				bplist, berr := bdict.(*Dictionary).postingsList([]byte(next.Term), nil, nil)
 				if aerr != berr {
 					rv = append(rv, fmt.Sprintf("field %s, term: %s, postingsList() errors different: %v %v",
 						fieldName, next.Term, aerr, berr))

index/scorch/segment/zap/segment.go

@@ -343,8 +343,9 @@ func (s *SegmentBase) DocNumbers(ids []string) (*roaring.Bitmap, error) {
 			return nil, err
 		}
 
+		var postings *PostingsList
 		for _, id := range ids {
-			postings, err := idDict.postingsList([]byte(id), nil)
+			postings, err = idDict.postingsList([]byte(id), nil, postings)
 			if err != nil {
 				return nil, err
 			}