Re: Support for 8-byte TOAST values (aka the TOAST infinite loop problem) - Mailing list pgsql-hackers

On Fri, Aug 08, 2025 at 03:32:16PM -0400, Tom Lane wrote:
> I found some time to look at the v4 patchset, and have a bunch of
> comments of different sizes.

Thanks for the comments.  I have replied to some of the items here:
https://www.postgresql.org/message-id/aJbygEBqJgmLS0wF%40paquier.xyz

Will try to answer the rest here.

> 0001:
>
> I'm good with widening all these values to 64 bits, but I wonder
> if it's a great idea to use unadorned "uint64" as the data type.
> That's impossible to grep for, doesn't convey anything much about
> what the variables are, etc.  I'm tempted to propose instead
> inventing a typedef "BigOid" or some such name (bikeshedding
> welcome).  The elog's could be handled with, say,
>     #define PRIBO PRIu64
> This suggestion isn't made with the idea that we'd someday switch
> to an even wider type, but just with the idea of making it clearer
> what these values are being used for.  When you see "Oid" you
> know it's some sort of object identifier, and I'm sad to give
> that up here.

Already mentioned on the other message, but I'm OK with a "bigoid"
type and a BigOid type.  Honestly, I'm not sure about a replacement
for PRIu64, as we don't really do that for Oids with %u.

> toast_pointer isn't initialized at this point.  I see you fixed that
> in 0004, but it doesn't help to split the patch series if the
> intermediate steps are broken.

Oops.  FWIW, I've rebased this patch set much more than 4 times.  It
looks like I've messed up some of the diffs.  Sorry about that.

> 0003:
>
> No objection to the struct renaming, but does this go far
> enough?  Aren't we going to need to rename TOAST_POINTER_SIZE
> to TOAST_OID_POINTER_SIZE, etc, so that we can have similar
> symbols for the wider version?  I'm suspicious of not renaming
> the functions that work on these, too.  (Oh, it looks like you
> did some of that in later parts.)

Yeah.  I've stuck that into the later parts where the int8 bits have
been added.  Perhaps I've not been ambitious enough.

> BTW, given that varatt.h has "typedef struct varatt_external {...}
> varatt_external", I'm certain that the vast majority of uses of
> "struct varatt_external" could be shortened to "varatt_external".
> And I think we should do that, because using "struct foo" not "foo"
> is not project style.  This patch would be a fine time to do that.

Good point.

> 0004:
>
> Shouldn't VARATT_EXTERNAL_GET_POINTER go away entirely?
> It looks to me like every use of that should be replaced by
> toast_external_info_get_data().

Point about indirect pointers mentioned on the other message, where we
could rename VARATT_EXTERNAL_GET_POINTER to
VARATT_EXTERNAL_INDIRECT_GET_POINTER or equivalent to limit the
confusion?

> I wonder if we shouldn't try to get rid of the phraseology "standard
> TOAST pointer", and instead write something like "short TOAST pointer"
> or "small TOAST pointer".  These aren't really going to be more
> "standard" than the wider ones, IMO.

I'm seeing one place in arrayfuncs.c and one in detoast.h using this
term on HEAD.  I would do simpler: no standard and no short, just with
a removal of the "standard" part if I were to change something.

> I don't like replacing "va_valueid" with just "value".  Dropping
> the "id" is not an improvement, because now a reader might be
> confused about whether this is somehow the actual value of the
> toasted datum.

Okay, sure.  One reason behind the field renaming was also to track
down all the areas in need to be updated.

> Functions in toast_external.c are under-documented.  Some lack
> any header comment whatever, and others have one that doesn't
> actually say what they do.

Okay.

> I kind of wonder whether the run-time indirection this design causes
> is going to be a performance problem.  Perhaps not, given the expenses
> involved in accessing a toasted value, but it has a faint feeling
> of overdesign.

Mentioned on the other message, linked to this message:
https://www.postgresql.org/message-id/aGzLiDUB_18-8aVQ%40paquier.xyz

> It looks like a lot of this code would just flat out dump core if
> passed an invalid vartag value.  Probably not good enough, given
> that we might look at corrupted data.

Right.  That's where we would need an error path in
toast_external_get_info() if nothing is found.  That's a cheap
defense, I guess.  Good thing is that the lookups are centralized in a
single code path, at least with this design.

> I kind of feel that treating this operation as a toast_external_info
> method is the wrong thing, since where it looks like you are going
> is to fetch an identifier and only then decide which vartag you
> need to use.  That ugliness comes out here:
>
> +    /*
> +     * Retrieve the external TOAST information, with the value still unknown.
> +     * We need to do this again once we know the actual value assigned, to
> +     * define the correct vartag_external for the new TOAST tuple.
> +     */

Yeah, I was feeling a bit depressed when writing the concept of what a
"default" method should be before assigning the value, but that was
still feeling right.

> 0007:
>
> Surely we do not need the cast in this:
>
> +    return murmurhash64((int64) DatumGetInt64(datum));
>
> I see you copied that from int4hashfast, but that's not good
> style either.

Okay.

>
> More generally, though, why do we need catcache support for int8?
> There aren't any caches on toast chunks, and I doubt we are going to
> introduce any.  Sure there might be a need for this down the road,
> but I don't see that this patch series is the time to add it.

I recall that this part was required for the value conflict lookups
and also the TOAST value retrievals, so we are going to need it.

> 0008:
>
> I totally hate the idea of introducing a GUC for this.  This should be
> user-transparent.  Or if it isn't user-transparent, a GUC is still
> the wrong thing; some kind of relation option would be more sensible.

Per other email, I'm not sure what you entirely mean here: should 8B
values be the default with existing TOAST OID values kept as they are
across upgrades?  Or something else?

> 0009:
>
> I do not love this either.  I think the right thing is just to widen
> the existing nextOid counter to 64 bits, and increment it in-memory as
> 64 bits, and then return either all 64 bits when a 64-bit Oid is asked
> for, or just the low-order 32 bits when a 32-bit OID is asked for
> (with appropriate hacking for 32-bit wraparound).  Having a separate
> counter will make it too easy for the same value to be produced by
> nearby calls to the 32-bit and 64-bit Oid generators, which is likely
> a bad thing, if only because of potential confusion.

Acknowledged on other message, fine my be.

> 0011:
>
> Not reviewing this yet, because I disagree with the basic design.
> I didn't look at the later patches either.

Without an agreement about the design choices related to the first
patches up to 0006, I doubt there this is any need to review any of
the follow-up patches yet because the choices of the first patches
influence the next patches in the series.  Thanks for the feedback!
--
Michael