Feedback-ID: i0fe9450f:Fastmail
Date: Tue, 29 Nov 2022 09:22:22 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Preventing indirection for IndexPageGetOpaque for known-size
 page special areas
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On Mon, Nov 28, 2022 at 05:31:50PM -0500, Tom Lane wrote:
> After looking through this thread, I side with Robert: we should reject
> the remainder of this patch.  It gives up page layout flexibility that
> we might want back someday.  Moreover, I didn't see any hard evidence
> offered that there's any actual performance gain, let alone such a
> compelling gain that we should give up that flexibility for it.

As far as I understand, we are talking about this one:
https://www.postgresql.org/message-id/CAEze2Wj9c0abW2aRbC8JzOuUdGurO5av6SJ2=
H83du6tM+Q1rHQ@mail.gmail.com
After a few months looking at it again, I cannot get much excited
about switching these routines from a logic where we look at the page
header to something where we'd rely on the opaque structure size.

I am wondering if it would be worth adding an AssertMacro() like in
this one, though:
https://www.postgresql.org/message-id/YkaP64JvZTMgcHtq@paquier.xyz

This still uses PageGetSpecialPointer() to retrieve the pointer area
=66rom the page header, but it also checks that we have a match with the
structure expected by the index AM.
--
Michael

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