Function execute_with 

pub fn execute_with(
    sel: &Select,
    resolve: &dyn Fn(&str) -> Result<Option<Box<dyn Relation>>, Error>,
    exec: JoinExec,
    pushdown: bool,
) -> Result<(Vec<OutCol>, Vec<Value>, Plan), Error>

As execute_explain, with predicate pushdown switchable.

The switch exists so differential tests can run the SAME query with and without the rewrite and compare. Without it, a test believing it exercised pushdown could be measuring the unoptimised path, and the equivalence suite would prove nothing — the same reason JoinExec can force a strategy.