Re: sequential scans that pick up only deleted records do not honor query cancel or timeout

Merlin Moncure <mmoncure@gmail.com> writes:
> Basically, $subject says it all.  It's pretty easy to reproduce:
> delete all the records from a large table and execute any sequentially
> scanning query before autocvacuum comes around and cleans the table
> up; the query will be uncancellable.  This can result in fairly
> pathological behavior in i/o constrained systems because the query
> will bog itself down writing out hint bits for minutes or hours
> without any way to cancel or effective i/o throttling (unlike vacuum).

> IMO, this should be backpatched, and is likely fixed by injecting an
> interrupts check at a strategic location.  But where? I was thinking
> in heapgetpage() but here are no checks elsehwere in heapam.c which is
> a red flag.

heapgetpage() seems like the most reasonable place to me, as there we'll
only be making the check once per page not once per tuple.

            regards, tom lane