Horton

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

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Eisentraut <peter@eisentraut.org>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, "Guo, Adam" <adamguo@amazon.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-05-03T14:13:43Z
Lists: pgsql-hackers 
Peter Eisentraut <peter@eisentraut.org> writes:
> On 30.04.24 19:29, Tom Lane wrote:
>> Also, the bigger picture here is the seeming assumption that "if
>> we change pg_trgm then it will be safe to replicate from x86 to
>> arm".  I don't believe that that's a good idea and I'm unwilling
>> to promise that it will work, regardless of what we do about
>> char signedness.  That being the case, I don't want to invest a
>> lot of effort in the signedness issue.  Option (1) is clearly
>> a small change with little if any risk of future breakage.

> But note that option 1 would prevent some replication that is currently 
> working.

The point of this thread though is that it's working only for small
values of "work".  People are rightfully unhappy if it seems to work
and then later they get bitten by compatibility problems.

Treating char signedness as a machine property in pg_control would
signal that we don't intend to make it work, and would ensure that
even the most minimal testing would find out that it doesn't work.

If we do not do that, it seems to me we have to buy into making
it work.  That would mean dealing with the consequences of an
incompatible change in pg_trgm indexes, and then going through
the same dance again the next time(s) similar problems are found.

			regards, tom lane

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.