func (this *builder) fastCount(node *algebra.Subselect) (bool, error) {
	if node.From() == nil ||
		node.Where() != nil ||
		node.Group() != nil {
		return false, nil
	}

	from, ok := node.From().(*algebra.KeyspaceTerm)
	if !ok ||
		from.Projection() != nil ||
		from.Keys() != nil {
		return false, nil
	}

	from.SetDefaultNamespace(this.namespace)
	keyspace, err := this.getTermKeyspace(from)
	if err != nil {
		return false, err
	}

	for _, term := range node.Projection().Terms() {
		count, ok := term.Expression().(*algebra.Count)
		if !ok || count.Operand() != nil {
			return false, nil
		}
	}

	scan := plan.NewCountScan(keyspace, from)
	this.children = append(this.children, scan)
	return true, nil
}

func (this *builder) visitFrom(node *algebra.Subselect, group *algebra.Group) error {
	count, err := this.fastCount(node)
	if err != nil {
		return err
	}

	if count {
		this.maxParallelism = 1
	} else if node.From() != nil {
		if this.where != nil || group != nil {
			this.limit = nil
		}

		_, err := node.From().Accept(this)
		if err != nil {
			return err
		}
	} else {
		// No FROM clause
		scan := plan.NewDummyScan()
		this.children = append(this.children, scan)
		this.maxParallelism = 1
	}

	return nil
}

func (this *builder) VisitSubselect(node *algebra.Subselect) (interface{}, error) {
	prevCover := this.cover
	prevCorrelated := this.correlated
	defer func() {
		this.cover = prevCover
		this.correlated = prevCorrelated
	}()

	this.correlated = node.IsCorrelated()

	if this.cover == nil {
		this.cover = node
	}

	aggs, err := allAggregates(node, this.order)
	if err != nil {
		return nil, err
	}

	this.where = node.Where()

	group := node.Group()
	if group == nil && len(aggs) > 0 {
		group = algebra.NewGroup(nil, nil, nil)
		this.where = constrainAggregate(this.where, aggs)
	}

	this.children = make([]plan.Operator, 0, 16)    // top-level children, executed sequentially
	this.subChildren = make([]plan.Operator, 0, 16) // sub-children, executed across data-parallel streams

	err = this.visitFrom(node, group)
	if err != nil {
		return nil, err
	}

	if this.coveringScan != nil {
		coverer := expression.NewCoverer(this.coveringScan.Covers())
		err = this.cover.MapExpressions(coverer)
		if err != nil {
			return nil, err
		}

		if this.where != nil {
			this.where, err = coverer.Map(this.where)
			if err != nil {
				return nil, err
			}
		}
	}

	if node.Let() != nil {
		this.subChildren = append(this.subChildren, plan.NewLet(node.Let()))
	}

	if node.Where() != nil {
		this.subChildren = append(this.subChildren, plan.NewFilter(node.Where()))
	}

	if group != nil {
		this.visitGroup(group, aggs)
	}

	projection := node.Projection()
	this.subChildren = append(this.subChildren, plan.NewInitialProject(projection))

	// Initial DISTINCT (parallel)
	if projection.Distinct() || this.distinct {
		this.subChildren = append(this.subChildren, plan.NewDistinct())
	}

	if !this.delayProjection {
		// Perform the final projection if there is no subsequent ORDER BY
		this.subChildren = append(this.subChildren, plan.NewFinalProject())
	}

	// Parallelize the subChildren
	this.children = append(this.children, plan.NewParallel(plan.NewSequence(this.subChildren...), this.maxParallelism))

	// Final DISTINCT (serial)
	if projection.Distinct() || this.distinct {
		this.children = append(this.children, plan.NewDistinct())
	}

	// Serialize the top-level children
	return plan.NewSequence(this.children...), nil
}

func allAggregates(node *algebra.Subselect, order *algebra.Order) (map[string]algebra.Aggregate, error) {
	aggs := make(map[string]algebra.Aggregate)

	if node.Let() != nil {
		for _, binding := range node.Let() {
			collectAggregates(aggs, binding.Expression())
			if len(aggs) > 0 {
				return nil, fmt.Errorf("Aggregates not allowed in LET.")
			}
		}
	}

	if node.Where() != nil {
		collectAggregates(aggs, node.Where())
		if len(aggs) > 0 {
			return nil, fmt.Errorf("Aggregates not allowed in WHERE.")
		}
	}

	group := node.Group()
	if group != nil {
		letting := group.Letting()
		for _, binding := range letting {
			collectAggregates(aggs, binding.Expression())
		}

		having := group.Having()
		if having != nil {
			collectAggregates(aggs, having)
		}
	}

	projection := node.Projection()
	if projection != nil {
		for _, term := range projection.Terms() {
			if term.Expression() != nil {
				collectAggregates(aggs, term.Expression())
			}
		}
	}

	if order != nil {
		allow := len(aggs) > 0

		for _, term := range order.Terms() {
			if term.Expression() != nil {
				collectAggregates(aggs, term.Expression())
			}
		}

		if !allow && len(aggs) > 0 {
			return nil, fmt.Errorf("Aggregates not available for this ORDER BY.")
		}
	}

	return aggs, nil
}