From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
cc: Hans Buschmann <buschmann@nidsa.net>,
        Amit Langote <amitlangote09@gmail.com>,
        "David G. Johnston" <david.g.johnston@gmail.com>,
        "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: AW: AW: BUG #18147: ERROR: invalid perminfoindex 0 in RTE with
 relid xxxxx
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Comments: In-reply-to Peter Geoghegan <pg@bowt.ie>
	message dated "Mon, 23 Oct 2023 11:49:20 -0700"
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Date: Mon, 23 Oct 2023 14:55:08 -0400
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Peter Geoghegan <pg@bowt.ie> writes:
> On Mon, Oct 23, 2023 at 11:30 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> If this is because it's "only a hint" and doesn't have to be reliable,
>> okay, but the documentation around indexUnchanged utterly fails to
>> make that clear.  I fear some poor index AM writer is going to get
>> screwed big time when they assume this flag is good for more than
>> heuristic decisions about when to do noncritical maintenance.

> That's fair, though note that index_unchanged_by_update does at least
> own the fact that it ignores the effects of BEFORE triggers in code
> comments. It also doesn't care about predicates in partial indexes,
> for reasons that are fairly specific to the way that the hint is
> actually used on the nbtree side.

Yeah, there are comments within index_unchanged_by_update about those
things.  What I'm unhappy about is that indexam.sgml's discussion of
the indexUnchanged flag makes it sound far more trustworthy than it
actually is.  Somebody who just read that doco and didn't scour the
underlying code would be badly misled.

			regards, tom lane