Re: case insensitive collation of Greek's sigma

On 26.11.21 08:37, Jakub Jedelsky wrote:
> postgres=# SELECT
> postgres-# 'ΣΣ' ILIKE 'σσ' COLLATE "en_US",
> postgres-# 'ΣΣ' ILIKE 'σς' COLLATE "en_US"
> postgres-# ;
>  ?column? | ?column?
> ----------+----------
>  t        | f
> (1 row)
>
> postgres=# SELECT
> postgres-# 'ΣΣ' ILIKE 'σσ' COLLATE "en-US-x-icu",
> postgres-# 'ΣΣ' ILIKE 'σς' COLLATE "en-US-x-icu";
>  ?column? | ?column?
> ----------+----------
>  f        | t
> (1 row)
>
> If I could start, I think both results are wrong as both should return
> True. If I got it right, in the background there is a lower() function
> running to compare strings, which is not enough for such cases (until
> the left side isn't taken as a standalone word).

The reason for these results is that for multibyte encodings, a ILIKE b
basically does lower(a) LIKE lower(b), and

select lower('ΣΣ' COLLATE "en_US"), lower('ΣΣ' COLLATE "en-US-x-icu");
lower | lower
-------+-------
σσ | σς

Running lower() like this is really the wrong thing to do. We should be
doing "case folding" instead, which normalizes these differences for the
purpose of case-insensitive comparisons.