Function multiSort

worker/sort.go:373–485

(ctx context.Context, r *sortresult, ts *pb.SortMessage)

}

func multiSort(ctx context.Context, r *sortresult, ts *pb.SortMessage) error {
	span := trace.SpanFromContext(ctx)
	span.SetAttributes(attribute.String("multiSort", "true"))

	// SrcUids for other queries are all the uids present in the response of the first sort.
	dest := destUids(r.reply.UidMatrix)

	// For each uid in dest uids, we have multiple values which belong to different attributes.
	// 1  -> [ "Alice", 23, "1932-01-01"]
	// 10 -> [ "Bob", 35, "1912-02-01" ]
	sortVals := make([][]types.Val, len(dest.Uids))
	for idx := range sortVals {
		sortVals[idx] = make([]types.Val, len(ts.Order))
	}

	seen := make(map[uint64]struct{})
	// Walk through the uidMatrix and put values for this attribute in sortVals.
	for i, ul := range r.reply.UidMatrix {
		x.AssertTrue(len(ul.Uids) == len(r.vals[i]))
		for j, uid := range ul.Uids {
			uidx := algo.IndexOf(dest, uid)
			x.AssertTrue(uidx >= 0)

			if _, ok := seen[uid]; ok {
				// We have already seen this uid.
				continue
			}
			seen[uid] = struct{}{}
			sortVals[uidx][0] = r.vals[i][j]
		}
	}

	// Execute rest of the sorts concurrently.
	och := make(chan orderResult, len(ts.Order)-1)
	for i := 1; i < len(ts.Order); i++ {
		in := &pb.Query{
			Attr:    ts.Order[i].Attr,
			UidList: dest,
			Langs:   ts.Order[i].Langs,
			ReadTs:  ts.ReadTs,
		}
		go fetchValues(ctx, in, i, och)
	}

	var oerr error
	// TODO - Verify behavior with multiple langs.
	for i := 1; i < len(ts.Order); i++ {
		or := <-och
		if or.err != nil {
			if oerr == nil {
				oerr = or.err
			}
			continue
		}

		result := or.r
		x.AssertTrue(len(result.ValueMatrix) == len(dest.Uids))
		for i := range dest.Uids {