Re: Damage control for planner's get_actual_variable_endpoint() runaway

On Mon, 21 Nov 2022 at 15:23, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Mon, Nov 21, 2022 at 10:14 AM Simon Riggs
> <simon(dot)riggs(at)enterprisedb(dot)com> wrote:
> > > > What we need is a solution that avoids reading an unbounded number of
> > > > tuples under any circumstances. I previously suggested using
> > > > SnapshotAny here, but Tom didn't like that. I'm not sure if there are
> > > > safety issues there or if Tom was just concerned about the results
> > > > being misleading. Either way, maybe there's some variant on that theme
> > > > that could work. For instance, could we teach the index scan to stop
> > > > if the first 100 tuples that it finds are all invisible? Or to reach
> > > > at most 1 page, or at most 10 pages, or something?
> > >
> > > A hard limit on the number of index pages examined seems like it
> > > might be a good idea.
> >
> > Good, that is what the patch does.
>
> <looks at patch>
>
> Oh, that's surprisingly simple. Nice!
>
> Is there any reason to tie this into page costs? I'd be more inclined
> to just make it a hard limit on the number of pages. I think that
> would be more predictable and less prone to surprising (bad) behavior.
> And to be honest I would be inclined to make it quite a small number.
> Perhaps 5 or 10. Is there a good argument for going any higher?

+1, that makes the patch smaller and the behavior more predictable.

(Just didn't want to do anything too simple, in case it looked like a kluge.)

--
Simon Riggs http://www.EnterpriseDB.com/