Function relational 

pub fn relational(s: &Select) -> bool

Whether a select needs the plan.

The boundary is which operators the query wants, not how big it is. A scan with a where and an order by is what crate::read has always answered directly and it keeps answering it — select count(*) included, whose whole point is that a maintained arrangement already knows its own size (docs/23 §23.19) and that compiling a plan to rediscover it would be work for nothing. What comes here is a query that relates, groups or deduplicates, because those are the three things the plan has operators for and this module has none.