Horton

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

Robert Haas <robertmhaas@gmail.com>

View thread View on postgr.es
From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, Simon Riggs <simon.riggs@enterprisedb.com>, pgsql-hackers@lists.postgresql.org, Jakub Wartak <jakub.wartak@enterprisedb.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2022-11-21T22:48:22Z
Lists: pgsql-hackers 
On Mon, Nov 21, 2022 at 5:15 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I think we should content ourselves with improving the demonstrated
> case, which is where we're forced to do a lot of heap fetches due
> to lots of not-all-visible tuples.  Whether we can spend a lot of
> time scanning the index without ever finding a tuple at all seems
> hypothetical.  Without more evidence of a real problem, I do not
> wish to inject warts as horrid as this one into the index AM API.

All right. I've been bitten by this problem enough that I'm a little
gun-shy about accepting anything that doesn't feel like a 100%
solution, but I admit that the scenario I described does seem a little
bit far-fetched.

I won't be completely shocked if somebody finds a way to hit it, though.

-- 
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.