Horton

Re: pg_trgm comparison bug on cross-architecture replication due to different char implementation

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: "Guo, Adam" <adamguo@amazon.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-04-30T03:15:20Z
Lists: pgsql-hackers

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On Tue, Apr 23, 2024 at 11:57 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> "Guo, Adam" <adamguo@amazon.com> writes:
> > I would like to report an issue with the pg_trgm extension on
> > cross-architecture replication scenarios. When an x86_64 standby
> > server is replicating from an aarch64 primary server or vice versa,
> > the gist_trgm_ops opclass returns different results on the primary
> > and standby.
>
> I do not think that is a supported scenario.  Hash functions and
> suchlike are not guaranteed to produce the same results on different
> CPU architectures.  As a quick example, I get
>
> regression=# select hashfloat8(34);
>  hashfloat8
> ------------
>    21570837
> (1 row)
>
> on x86_64 but
>
> postgres=# select hashfloat8(34);
>  hashfloat8
> ------------
>  -602898821
> (1 row)
>
> on ppc32 thanks to the endianness difference.
>
> > Given that this has problem has come up before and seems likely to
> > come up again, I'm curious what other broad solutions there might be
> > to resolve it?
>
> Reject as not a bug.  Discourage people from thinking that physical
> replication will work across architectures.

While cross-arch physical replication is not supported, I think having
architecture dependent differences is not good and It's legitimate to
fix it. FYI the 'char' data type comparison is done as though char is
unsigned. I've attached a small patch to fix it. What do you think?

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

Commits

  1. pg_upgrade: Check for the expected error message in TAP tests.

  2. Fix a typo in 005_char_signedness.pl test.

  3. Add test 005_char_signedness.pl to meson.build.

  4. Fix an issue with index scan using pg_trgm due to char signedness on different architectures.

  5. pg_upgrade: Add --set-char-signedness to set the default char signedness of new cluster.

  6. pg_upgrade: Preserve default char signedness value from old cluster.

  7. pg_resetwal: Add --char-signedness option to change the default char signedness.

  8. Add default_char_signedness field to ControlFileData.

  9. Remove unneeded nbtree array preprocessing assert.