Re: case insensitive collation of Greek's sigma

I realise this may not be applicable to the original problem, but
non-deterministic collations seems to offer a solution::

dakkar(at)[local] dakkar=> create collation "en-US-ins-icu" (
provider=icu,
locale='en-US-u-ks-level2',
deterministic=false
);

dakkar(at)[local] dakkar=> select 'ΣΣ' = 'σσ' collate "en-US-ins-icu";
┌──────────┐
│ ?column? │
├──────────┤
│ t │
└──────────┘
(1 row)

dakkar(at)[local] dakkar=> select 'ΣΣ' = 'σς' collate "en-US-ins-icu";
┌──────────┐
│ ?column? │
├──────────┤
│ t │
└──────────┘
(1 row)

dakkar(at)[local] dakkar=> select 'ΣΣ' = 'α' collate "en-US-ins-icu";
┌──────────┐
│ ?column? │
├──────────┤
│ f │
└──────────┘
(1 row)

Notice, though:

* I don't understand what that ``-u-`` is doing in ``locale``, but
it's necessary
* as the docs
https://www.postgresql.org/docs/13/collation.html#COLLATION-NONDETERMINISTIC
say:

- B-tree cannot use deduplication with indexes that use a
nondeterministic collation
- certain operations are not possible with nondeterministic
collations, such as pattern matching operations (this means you
can't use ``LIKE``)

--
Dakkar - <Mobilis in mobile>
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