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Date: Mon, 22 May 2023 14:27:10 +0200
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Subject: Re: Order changes in PG16 since ICU introduction
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To: Jeff Davis <pgsql@j-davis.com>, "Jonathan S. Katz"
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Cc: Tom Lane <tgl@sss.pgh.pa.us>, Regina Obe <lr@pcorp.us>,
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From: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
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On 18.05.23 19:55, Jeff Davis wrote:
> On Wed, 2023-05-17 at 19:59 -0400, Jonathan S. Katz wrote:
>> I did a quicker read through this time. LGTM overall. I like what you
>> did with the explanations around sensitivity (now it makes sense).
> 
> Committed, thank you.
> 
> There are a few things I don't understand that would be good to
> document better:
> 
> * Rules. I still don't quite understand the use case: are these for
> people inventing new languages? What is a plausible use case that isn't
> covered by the existing locales and collation settings? Do rules make
> sense for a database default collation? Are they for language experts
> only or might an ordinary developer benefit from using them?

The rules are for setting whatever sort order you like.  Maybe you want 
to sort + before - or whatever.  It's like, if you don't like it, build 
your own.

> * The collation types "phonebk", "emoji", etc.: are these variants of
> particular locales, or do they make sense in multiple locales? I don't
> know where they fit in or how to document them.

The k* settings are parametric settings, in that they transform the sort 
key in some algorithmic way.  The co settings are just everything else. 
They are not parametric, they are just some other sort order that 
someone spelled out explicitly.

> * I don't understand what "kc" means if "ks" is not set to "level1".

There is an example here: 
https://peter.eisentraut.org/blog/2023/05/16/overview-of-icu-collation-settings#colcaselevel