func (this *Context) EvaluateSubquery(query *algebra.Select, parent value.Value) (value.Value, error) {
subresults := this.getSubresults()
subresult, ok := subresults.get(query)
if ok {
return subresult.(value.Value), nil
}
subplans := this.getSubplans()
subplan, planFound := subplans.get(query)
if !planFound {
var err error
subplan, err = planner.Build(query, this.datastore, this.systemstore, this.namespace, true)
if err != nil {
return nil, err
}
// Cache plan
subplans.set(query, subplan)
}
pipeline, err := Build(subplan.(plan.Operator), this)
if err != nil {
return nil, err
}
// Collect subquery results
collect := NewCollect()
sequence := NewSequence(pipeline, collect)
sequence.RunOnce(this, parent)
// Await completion
ok = true
for ok {
_, ok = <-collect.Output().ItemChannel()
}
results := collect.ValuesOnce()
// Cache results
if !planFound && !query.Subresult().IsCorrelated() {
subresults.set(query, results)
}
return results, nil
}
func (this *builder) VisitSelect(stmt *algebra.Select) (interface{}, error) {
// Restore previous values when exiting. VisitSelect()
// can be called multiple times by set operators
prevCover := this.cover
prevOrder := this.order
prevLimit := this.limit
prevProjection := this.delayProjection
defer func() {
this.cover = prevCover
this.order = prevOrder
this.limit = prevLimit
this.delayProjection = prevProjection
}()
order := stmt.Order()
offset := stmt.Offset()
limit := stmt.Limit()
this.order = order
if order != nil {
// If there is an ORDER BY, delay the final projection
this.delayProjection = true
this.cover = stmt
} else {
this.cover = nil
}
if order != nil || offset != nil {
this.limit = nil
} else if limit != nil {
this.limit = limit
}
sub, err := stmt.Subresult().Accept(this)
if err != nil {
return nil, err
}
if order == nil && offset == nil && limit == nil {
return sub, nil
}
children := make([]plan.Operator, 0, 5)
children = append(children, sub.(plan.Operator))
if order != nil {
children = append(children, plan.NewOrder(order))
}
if offset != nil {
children = append(children, plan.NewOffset(offset))
}
if limit != nil {
children = append(children, plan.NewLimit(limit))
}
// Perform the delayed final projection now, after the ORDER BY
if this.delayProjection {
children = append(children, plan.NewFinalProject())
}
return plan.NewSequence(children...), nil
}