scheduler() and aync_scheduler() on QueryContext

[Li Jin]

Hi,

I am reading Acero and got confused about the use of
QueryContext::scheduler() and QueryContext::async_scheduler(). So I have a
couple of questions:

(1) What are the different purposes of these two?
(2) Does scheduler/aysnc_scheduler own any threads inside their respective
classes or do they use the thread pool from ExecContext::executor()?

Thanks,
Li