EBCDIC sorting as a use case for ICU rules

 Hi,

In the "Order changes in PG16 since ICU introduction" discussion, one
sub-thread [1] was about having a credible use case for tailoring collations
with custom rules, a new feature in v16.

At a conference this week I was asked if ICU could be able to
sort like EBCDIC [2]. It turns out it has been already	asked on
-general a few years ago [3] with no satisfactory answer at the time ,
and that it can be implemented with rules in v16.

A collation like the following this seems to work (the rule simply enumerates
US-ASCII letters in the EBCDIC alphabet order, with adequate quoting)

CREATE COLLATION ebcdic (provider='icu', locale='und',
rules=$$&'
'<'.'<'<'<'('<'+'<\|<'&'<'!'<'$'<'*'<')'<';'<'-'<'/'<','<'%'<'_'<'>'<'?'<'`'<':'<'#'<'@'<\'<'='<'"'?`:#@'="abcdefghijklmnopqr~stuvwxyz[^]{ABCDEFGHI}JKLMNOPQR\ST
UVWXYZ0123456789

Maybe this example could be added to the documentation except for
the problem that the rule is very long and dollar-quoting cannot be split
into several lines. Literals enclosed by single quotes can be split that
way, but would require escaping the single quotes in the rule, which
would lead to scary-looking over-quoted contents.

I'm open to suggestions on whether this EBCDIC example is worth being in the
doc in some form or putting this in the wiki would be good enough.



[1]
https://www.postgresql.org/message-id/flat/a28aba5fa6bf1abfff96e40b6d6acff8412edb15.camel%40j-davis.com

[2] https://en.wikipedia.org/wiki/EBCDIC

[3]
https://www.postgresql.org/message-id/flat/0A3221C70F24FB45833433255569204D1F84A7AD%40G01JPEXMBYT05


Best regards,
-- 
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite