On 1/4/22, 2:35 PM, "Stephen Frost" <[email protected]> wrote:
>>
>> Not saying that I've got any idea how to fix that case offhand, and we
>> don't really support such a thing today as the server would just stop
>> instead, ...
>> Perhaps that's a
>> worthwhile tradeoff for being able to generally avoid having to vacuum
>> and deal with transaction wrap-around, but I have to wonder if there
>> might be a better answer.
>>
For the target use cases that PostgreSQL is designed for, it's a very
worthwhile tradeoff in my opinion. Such long-running transactions need to be
killed.
Re: -- If after upgrade page has no free space for special data, tuples are
converted to "double xmax" format: xmin became virtual
FrozenTransactionId, xmax occupies the whole 64bit.
Page converted to new format when vacuum frees enough space.
I'm concerned about the maintainability impact of having 2 new on-disk page
formats. It's already complex enough with XIDs and multixact-XIDs.
If the lack of space for the two epochs in the special data area is a problem
only in an upgrade scenario, why not resolve the problem before completing the
upgrade process like a kind of post-process pg_repack operation that converts
all "double xmax" pages to the "double-epoch" page format? i.e. maybe the
"double xmax" representation is needed as an intermediate representation during
upgrade, but after upgrade completes successfully there are no pages with the
"double-xmax" representation. This would eliminate a whole class of coding
errors and would make the code dealing with 64-bit XIDs simpler and more
maintainable.
/Jim
