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In-Reply-To: <2be31f17-5405-4de9-8d73-90ebc322f7d8@vondra.me>
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Wed, 25 Mar 2026 14:54:16 -0400
Message-ID: 
 <CAAKRu_Ypa7-JGVR+fstDxU5Cfitk_rf5ijdaqwtoPkztursufA@mail.gmail.com>
Subject: Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM
 on-access)
To: Tomas Vondra <tomas@vondra.me>
Cc: Andres Freund <andres@anarazel.de>,
 Kirill Reshke <reshkekirill@gmail.com>,
	Chao Li <li.evan.chao@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>,
	Xuneng Zhou <xunengzhou@gmail.com>, Robert Haas <robertmhaas@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
 Heikki Linnakangas <hlinnaka@iki.fi>
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On Wed, Mar 25, 2026 at 2:02=E2=80=AFPM Tomas Vondra <tomas@vondra.me> wrot=
e:
>
> 0002
>
> - Don't we usually keep "flags" as the last parameter? It seems a bit
> weird that it's added in between relation and snapshot.

In an earlier review, Andres said he disliked using flags as the last
parameter for index_beginscan() because its current last two
parameters are integers (nkeys and norderbys), which could be
confusing. Personally, I think you have to look at the function
signature before just randomly passing stuff, and so it shouldn't
matter -- but I didn't care enough to argue. If you agree with me that
they should be last, then it's two against one and I'll change it back
:) I can keep the callsite comments naming the flags parameter.

> - Do we really want to pass two sets of flags to table_beginscan_common?
>  I realize it's done to ensure "users" don't use internal flags, but
> then maybe it'd be better to do that check in the places calling the
> _common? Someone adding a new caller can break this in various ways
> anyway, e.g. by setting bits in the internal flags, no?

Yes, callers of table_beginscan_common() could pass flags they
shouldn't in internal_flags. But I was mostly trying to prevent the
case where a user picks a flag that overlaps with an internal flag,
conditionally passes it as a user flag, and then when they test for it
in their AM-specific code, they aren't actually checking if their own
flag is set.

Anyway, it's not hard to move:
    Assert((flags & SO_INTERNAL_FLAGS) =3D=3D 0);
into the table_beginscan_common() callers and then pass the internal
flags the caller wants to pass + the user specified flags to
table_beginscan_common(). And I think that fixes what you are talking
about?

> If we want to have these checks, should we be more thorough? Should we
> check the internal flags only set internal flags?

That's easy enough too.
Assert((internal_flags & ~SO_INTERNAL_FLAGS) =3D=3D 0); I think does the tr=
ick.

I think this would largely be the same as having
table_beginscan_common() callers validate that the user-passed flags
are not internal and then OR them together with the internal flags
they want to pass to table_beginscan_common().

I'm trying to think of cases where the two approaches would differ so
I can decide which to do.

> 0003
>
> - Half the "beginscan" calls use a ternary operator directly, half sets
> a variable first (and then uses that). Often mixed in the same file.
> Shouldn't it be a bit consistent?

Indeed.

- Melanie