func (this *builder) VisitSelect(stmt *algebra.Select) (interface{}, error) {
order := stmt.Order()
offset := stmt.Offset()
limit := stmt.Limit()
delayProjection := this.delayProjection
// If there is an ORDER BY, delay the final projection
if order != nil {
this.order = order
this.delayProjection = true
}
sub, err := stmt.Subresult().Accept(this)
if err != nil {
return nil, err
}
if order == nil && offset == nil && limit == nil {
return sub, nil
}
children := make([]Operator, 0, 5)
children = append(children, sub.(Operator))
if order != nil {
if this.order == nil {
// Disallow aggregates in ORDER BY
aggs := make(map[string]algebra.Aggregate)
for _, term := range order.Terms() {
collectAggregates(aggs, term.Expression())
if len(aggs) > 0 {
return nil, fmt.Errorf("Aggregates not available for this ORDER BY.")
}
}
}
children = append(children, NewOrder(order))
}
if offset != nil {
children = append(children, NewOffset(offset))
}
if limit != nil {
children = append(children, NewLimit(limit))
}
// Perform the delayed final projection now, after the ORDER BY
if this.delayProjection {
children = append(children, NewParallel(NewFinalProject()))
this.delayProjection = delayProjection
}
return NewSequence(children...), nil
}
