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
}
