Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

On 3/25/26 19:54, Melanie Plageman wrote:
> On Wed, Mar 25, 2026 at 2:02 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>>
>> 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.
>

Who am I to argue with Andres? ;-) I'm kinda used to flags being the
last argument, but it's not something I'm particularly attached to.

>> - 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.
>

Ah, so we expect people to invent their "own" flags, outside what's in
ScanOptions? Or do I misunderstand how it works? (I admit not reading
the whole massive thread, as I was only interested in using the flags in
my own patch.)

> Anyway, it's not hard to move:
> Assert((flags & SO_INTERNAL_FLAGS) == 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?
>

Right. I wouldn't say it "fixes" it, because it wasn't a bug. But it
does ensure the two sets do not "overlap", which I assume should never
happen.

>> 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) == 0); I think does the trick.
>
> 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.
>

OK

--
Tomas Vondra