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

On 3/26/26 15:51, Melanie Plageman wrote:
> On Wed, Mar 25, 2026 at 7:29 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>>
>>>> - 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.)
>
> Yes, this isn't really explored in the rest of the thread. I thought
> since the flags are threaded all the way through and they can
> set/check the flags in the table AM-specific layer, it would make
> sense that they could choose flags for their own purposes. They don't
> have to wait for consensus on getting a new SO type added. I don't
> know if this is a bad idea. However, changing the table AM wrappers
> seems more justifiable if we are making them extensible in this way.
>

No idea. Do we have an example of a TAM actually needing this? If not,
I'd probably advise to remove that and keep the patch simpler. My past
attempts to future-proof a patch like this rarely worked.

If we want to give TAMs the opportunity to define custom flags, do we
already do something like that elsewhere? Is there a precedent how to do
that? If we allow the TAM to pick arbitrary flag values, it's easy to
end up with collisions later (if we add a new internal flag). Maybe
there is a way to prevent that? E.g. we could restrict internal flags to
0x0000FFFF, and custom flags to 0xFFFF0000?

regards

--
Tomas Vondra