Horton

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

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Simon Riggs <simon.riggs@enterprisedb.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Jakub Wartak <jakub.wartak@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2022-11-21T15:35:46Z
Lists: pgsql-hackers 
On Mon, Nov 21, 2022 at 10:32 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > 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.
>
> Agreed, a simple limit of N pages fetched seems appropriate.
>
> > 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?
>
> Sure: people are not complaining until it gets into the thousands.
> And you have to remember that the entire mechanism exists only
> because of user complaints about inaccurate estimates.  We shouldn't
> be too eager to resurrect that problem.
>
> I'd be happy with a limit of 100 pages.

OK.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. YA attempt at taming worst-case behavior of get_actual_variable_range.

  2. Improve performance of get_actual_variable_range with recently-dead tuples.

  3. Use SnapshotDirty rather than an active snapshot to probe index endpoints.