Method applyOrderAndPagination

query/query.go:2511–2578

applyOrderAndPagination orders each posting list by a given attribute before applying pagination.

(ctx context.Context)

// applyOrderAndPagination orders each posting list by a given attribute
// before applying pagination.
func (sg *SubGraph) applyOrderAndPagination(ctx context.Context) error {
	if len(sg.Params.Order) == 0 && len(sg.Params.FacetsOrder) == 0 {
		return nil
	}

	sg.updateUidMatrix()

	// See if we need to apply order based on facet.
	if len(sg.Params.FacetsOrder) != 0 {
		return sg.sortAndPaginateUsingFacet(ctx)
	}

	for _, it := range sg.Params.NeedsVar {
		// TODO(pawan) - Return error if user uses var order with predicates.
		if len(sg.Params.Order) > 0 && it.Name == sg.Params.Order[0].Attr &&
			(it.Typ == dql.ValueVar) {
			// If the Order name is same as var name and it's a value variable, we sort using that variable.
			return sg.sortAndPaginateUsingVar(ctx)
		}
	}

	// Todo: fix offset for cascade queries.
	if sg.Params.Count == 0 {
		// Only retrieve up to 1000 results by default.
		sg.Params.Count = 1000
	}

	x.AssertTrue(len(sg.Params.Order) > 0)

	ns, err := x.ExtractNamespace(ctx)
	if err != nil {
		return errors.Wrapf(err, "While ordering and paginating")
	}
	order := sg.createOrderForTask(ns)
	sortMsg := &pb.SortMessage{
		Order:     order,
		UidMatrix: sg.uidMatrix,
		Offset:    int32(sg.Params.Offset),
		Count:     int32(sg.Params.Count),
		ReadTs:    sg.ReadTs,
	}
	result, err := worker.SortOverNetwork(ctx, sortMsg)
	if err != nil {
		return err
	}

	x.AssertTrue(len(result.UidMatrix) == len(sg.uidMatrix))
	if sg.facetsMatrix != nil {
		// The order of uids in the lists which are part of the uidMatrix would have been changed
		// after sort. We want to update the order of lists in the facetMatrix accordingly.
		for idx, rl := range result.UidMatrix {
			fl := make([]*pb.Facets, 0, len(sg.facetsMatrix[idx].FacetsList))
			for _, uid := range rl.Uids {
				// Find index of this uid in original sorted uid list.
				oidx := algo.IndexOf(sg.uidMatrix[idx], uid)
				// Find corresponding facet.
				fl = append(fl, sg.facetsMatrix[idx].FacetsList[oidx])
			}