Re: pgsql: Handle the "und" locale in ICU versions 54 and older.

On Thu, 2023-03-23 at 13:25 -0400, Tom Lane wrote:
> Hmm, should we back-patch this?  Seems like existing branches would
> be even more at risk than v16, because more likely to be built with
> old ICU.  OTOH, we do also run the risk of breaking installations
> that weren't broken before.

I wondered the same thing[1] but ultimately figured the risk outweighed
the reward. My reasoning (which I didn't post before) was:

If a user currently has a collation with locale 'und', and ICU <= 54,
they are getting their actual locale from the environment. If we
backpatch, it will silently change their locale to be the root locale,
which could be different, and break their indexes. That seems too
dangerous for a minor release.

For a major release it's more tolerable to put something like that in
the release notes.

Fortunately, I think most users now are probably using the built-in
collations, or using the empty string before an "@" to specify the root
locale, which works in all ICU versions. Users would only specify it as
"und" if they happen to know about language tags.

Regards,
    Jeff Davis

[1]
https://www.postgresql.org/message-id/9afa6dbe0d31053ad265aeba488fde784fd5b7ab.camel@j-davis.com