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help / color / mirror / Atom feedFrom: lin teletele <[email protected]>
To: Shinya Kato <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: Álvaro Herrera <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Use pg_current_xact_id() instead of deprecated txid_current()
Date: Thu, 28 May 2026 01:08:46 +0800
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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 — 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 = 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 = pg_abs_s64(delta);
bool overflow;
if (delta >= 0)
overflow = pg_add_u64_overflow(val, abs_delta, &result);
else
overflow = pg_sub_u64_overflow(val, abs_delta, &result);
if (overflow)
ereport(ERROR, ...);
And xid8mi symmetrically (add/sub swapped):
uint64 abs_delta = pg_abs_s64(delta);
bool overflow;
if (delta >= 0)
overflow = pg_sub_u64_overflow(val, abs_delta, &result);
else
overflow = 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 == 2^63 (e.g. '0'::xid8 -
'9223372036854775808'::xid8). When val2 - val1 == 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 = val2 - val1;
if (diff > (uint64) PG_INT64_MAX + 1)
ereport(ERROR, ...);
/* diff == 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 PM Shinya Kato <[email protected]> wrote:
>
> On Thu, May 14, 2026 at 9:31 PM Shinya Kato <[email protected]> wrote:
> >
> > Rebased the patches.
>
> Rebased the patches, again.
>
> --
> Best regards,
> Shinya Kato
> NTT OSS Center
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Subject: Re: Use pg_current_xact_id() instead of deprecated txid_current()
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