From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Robert Haas <robertmhaas@gmail.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Catalog invalidations vs catalog scans vs ScanPgRelation()
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 09 Apr 2020 15:32:49 -0700"
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Date: Thu, 09 Apr 2020 18:52:32 -0400
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Andres Freund <andres@anarazel.de> writes:
> On 2020-04-09 16:56:03 -0400, Robert Haas wrote:
>> That seems like a fairly magical coding rule that will happen to work
>> in most practical cases but isn't really a principled approach to the
>> problem.

> I'm not sure it'd be that magical to only release resources at
> CommitTransactionCommand() time. We kinda do that for a few other things
> already.

I'd be worried about consumption of resources during a long transaction.
But maybe we could release at CommandCounterIncrement?

Still, I tend to agree with Robert that associating a snap with an
open catalog scan is the right way.  I have vague memories that a long
time ago, all catalog modifications were done via the fetch-from-a-
scan-and-update approach.  Starting from a catcache tuple instead
is a relative newbie.

If we're going to forbid using a catcache tuple as the starting point
for updates, one way to enforce it would be to have the catcache
not save the TID.

			regards, tom lane