From: | Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Damage control for planner's get_actual_variable_endpoint() runaway |
Date: | 2022-11-21 15:30:31 |
Message-ID: | CANbhV-H_M_FhX5C-2sMCPcK+wyr9W9H2_=8gCt4o-e4tZScngw@mail.gmail.com |
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Lists: | pgsql-hackers |
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/
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