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From: lin teletele <teletele.lin@gmail.com>
Date: Thu, 28 May 2026 01:08:46 +0800
Message-ID: 
 <CAP--GgNX4t-1DASBTmu+RvuZKW3Nb4-wV0PrF-Po95QwJ95wmA@mail.gmail.com>
Subject: Re: Use pg_current_xact_id() instead of deprecated txid_current()
To: Shinya Kato <shinya11.kato@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 =?UTF-8?Q?=C3=81lvaro_Herrera?= <alvherre@kurilemu.de>,
	pgsql-hackers <pgsql-hackers@lists.postgresql.org>
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Hi Shinya,

Thanks for the patches. I read v4-0001 and have a few small observations
from going through the arithmetic functions. I tested the suggestions
below locally on top of v4 =E2=80=94 they pass the existing xid regression =
tests
and produce identical output on the boundary cases listed in section 3.

1. xid8pl / xid8mi could reuse the helpers in common/int.h
----------------------------------------------------
xid8pl currently rolls its own overflow detection on a mixed-sign
addition:

result =3D val + (uint64) delta;
if ((delta > 0 && result < val) || (delta < 0 && result > val))
    ereport(ERROR, ...);

This is correct, but it's the only place in the tree that takes this
approach, and the (uint64)-of-a-signed-value plus sign-aware compare
takes a moment to convince oneself of. common/int.h already provides
pg_add_u64_overflow / pg_sub_u64_overflow, plus pg_abs_s64 which returns
uint64 and explicitly handles INT64_MIN, so xid8pl could be written as:

uint64 abs_delta =3D pg_abs_s64(delta);
bool overflow;

if (delta >=3D 0)
    overflow =3D pg_add_u64_overflow(val, abs_delta, &result);
else
    overflow =3D pg_sub_u64_overflow(val, abs_delta, &result);

if (overflow)
    ereport(ERROR, ...);

And xid8mi symmetrically (add/sub swapped):

uint64 abs_delta =3D pg_abs_s64(delta);
bool overflow;

if (delta >=3D 0)
    overflow =3D pg_sub_u64_overflow(val, abs_delta, &result);
else
    overflow =3D pg_add_u64_overflow(val, abs_delta, &result);

if (overflow)
    ereport(ERROR, ...);

This keeps the code inside the standard PG overflow-check idiom, and as
a side effect handles a delta of INT64_MIN cleanly (pg_abs_s64 returns
2^63 in that case without invoking UB).

2. INT64_MIN boundary in xid8_mi_xid8
----------------------------------------------------
In the val1 < val2 branch:

if (val2 - val1 > (uint64) PG_INT64_MAX + 1)
    ereport(ERROR, ...);
PG_RETURN_INT64(-((int64) (val2 - val1)));

The bound permits val2 - val1 =3D=3D 2^63 (e.g. '0'::xid8 -
'9223372036854775808'::xid8). When val2 - val1 =3D=3D 2^63, the cast
(int64)(val2 - val1) is implementation-defined (the value doesn't fit in
int64), and -INT64_MIN is signed overflow (UB). In practice on
two's-complement targets the answer comes out as INT64_MIN, which is the
correct value, but it relies on UB.

Pulling out the boundary explicitly keeps the same observable behavior
without the UB:

uint64 diff =3D val2 - val1;

if (diff > (uint64) PG_INT64_MAX + 1)
    ereport(ERROR, ...);
/* diff =3D=3D 2^63 maps to INT64_MIN */
if (diff > (uint64) PG_INT64_MAX)
    PG_RETURN_INT64(PG_INT64_MIN);
PG_RETURN_INT64(-(int64) diff);

3. Test coverage
----------------------------------------------------
The regression tests in xid.sql exercise the positive overflow side
nicely but miss a few boundaries on the negative side:

-- xid8 - xid8 at the INT64_MIN boundary (#2 above)
select '0'::xid8 - '9223372036854775808'::xid8;

-- xid8 + int8 / xid8 - int8 with INT64_MAX / INT64_MIN deltas
select '0'::xid8 + 9223372036854775807::bigint;
select '0'::xid8 - (-9223372036854775807 - 1)::bigint;
select '9223372036854775807'::xid8 - (-9223372036854775807 - 1)::bigint;

It would be good to pin those down in the expected output.

4. Documentation
----------------------------------------------------
v4-0001 adds four user-visible operators but doesn't touch doc/src/sgml/.
pg_lsn's arithmetic operators are documented in datatype.sgml around the
"pg_lsn Type" section -- it would be nice for the new xid8 operators to
get analogous coverage in the nearby xid8 paragraph.

As a separate observation (probably better as a follow-up thread rather
than expanding the scope of this one): xid8 currently has only hash and
btree opclasses, no BRIN. Since xid8 is strictly monotonic and never
wraps, BRIN minmax looks like a natural fit -- I'll raise that
separately if there's interest.

Regards,
Teletele

On Mon, May 25, 2026 at 4:10=E2=80=AFPM Shinya Kato <shinya11.kato@gmail.co=
m> wrote:
>
> On Thu, May 14, 2026 at 9:31=E2=80=AFPM Shinya Kato <shinya11.kato@gmail.=
com> wrote:
> >
> > Rebased the patches.
>
> Rebased the patches, again.
>
> --
> Best regards,
> Shinya Kato
> NTT OSS Center