perf(core): Use Intersect to Narrow Iterate Range and Reduce Memory Allocation in pl.Uids()

posting/list.go

@@ -1709,45 +1709,80 @@ func (l *List) Uids(opt ListOptions) (*pb.List, error) {
 	if opt.First == 0 {
 		opt.First = math.MaxInt32
 	}
-	// Pre-assign length to make it faster.
-	l.RLock()
-	// Use approximate length for initial capacity.
-	res := make([]uint64, 0, l.mutationMap.len()+codec.ApproxLen(l.plist.Pack))
-	out := &pb.List{}
-	if l.mutationMap.len() == 0 && opt.Intersect != nil && len(l.plist.Splits) == 0 {
-		if opt.ReadTs < l.minTs {
-			l.RUnlock()
-			return out, errors.Wrapf(ErrTsTooOld, "While reading UIDs")
+
+	getUidList := func() (*pb.List, error, bool) {
+		// Pre-assign length to make it faster.
+		l.RLock()
+		defer l.RUnlock()
+		// Use approximate length for initial capacity.
+		res := make([]uint64, 0, l.ApproxLen())
+		out := &pb.List{}
+
+		if opt.Intersect != nil && len(opt.Intersect.Uids) < l.ApproxLen() {
+			for _, uid := range opt.Intersect.Uids {
+				ok, _, err := l.findPosting(uid, opt.ReadTs)
+				if err != nil {
+					return nil, err, false
+				}
+				if ok {
+					res = append(res, uid)
+				}
+			}
+			out.Uids = res
+			return out, nil, false
 		}
-		algo.IntersectCompressedWith(l.plist.Pack, opt.AfterUid, opt.Intersect, out)
-		l.RUnlock()
-		return out, nil
-	}
 
-	err := l.iterate(opt.ReadTs, opt.AfterUid, func(p *pb.Posting) error {
-		if p.PostingType == pb.Posting_REF {
-			res = append(res, p.Uid)
-			if opt.First < 0 {
-				// We need the last N.
-				// TODO: This could be optimized by only considering some of the last UidBlocks.
-				if len(res) > -opt.First {
-					res = res[1:]
+		if l.mutationMap.len() == 0 && opt.Intersect != nil && len(l.plist.Splits) == 0 {
+			if opt.ReadTs < l.minTs {
+				return out, errors.Wrapf(ErrTsTooOld, "While reading UIDs"), false
+			}
+			algo.IntersectCompressedWith(l.plist.Pack, opt.AfterUid, opt.Intersect, out)
+			return out, nil, false
+		}
+
+		var uidMin, uidMax uint64 = 0, 0
+		if opt.Intersect != nil && len(opt.Intersect.Uids) > 0 {
+			uidMin = opt.Intersect.Uids[0]
+			uidMax = opt.Intersect.Uids[len(opt.Intersect.Uids)-1]
+		}
+
+		err := l.iterate(opt.ReadTs, opt.AfterUid, func(p *pb.Posting) error {
+			if p.PostingType == pb.Posting_REF {
+				if p.Uid < uidMin {
+					return nil
+				}
+				if p.Uid > uidMax && uidMax > 0 {
+					return ErrStopIteration
+				}
+				res = append(res, p.Uid)
+
+				if opt.First < 0 {
+					// We need the last N.
+					// TODO: This could be optimized by only considering some of the last UidBlocks.
+					if len(res) > -opt.First {
+						res = res[1:]
+					}
+				} else if len(res) > opt.First {
+					return ErrStopIteration
 				}
-			} else if len(res) > opt.First {
-				return ErrStopIteration
 			}
+			return nil
+		})
+		if err != nil {
+			return out, errors.Wrapf(err, "cannot retrieve UIDs from list with key %s",
+				hex.EncodeToString(l.key)), false
 		}
-		return nil
-	})
-	l.RUnlock()
-	if err != nil {
-		return out, errors.Wrapf(err, "cannot retrieve UIDs from list with key %s",
-			hex.EncodeToString(l.key))
+		out.Uids = res
+		return out, nil, true
 	}
 
 	// Do The intersection here as it's optimized.
-	out.Uids = res
-	lenBefore := len(res)
+	out, err, applyIntersectWith := getUidList()
+	if err != nil || !applyIntersectWith {
+		return out, err
+	}
+
+	lenBefore := len(out.Uids)
 	if opt.Intersect != nil {
 		algo.IntersectWith(out, opt.Intersect, out)
 	}