Thread

1. BUG #19004: Incorrect lowercasing of word-final Greek capital Sigma (Σ)

   PG Bug reporting form <noreply@postgresql.org> — 2025-07-31T03:06:10Z

   The following bug has been logged on the website:
   
   Bug reference:      19004
   Logged by:          Bosheng Peng
   Email address:      221250180@smail.nju.edu.cn
   PostgreSQL version: 17.5
   Operating system:   windows with docker
   Description:        
   
   When applying UPPER() to the Greek word 'κόσμος', PostgreSQL returns
   'ΚΌΣΜΟΣ'. However, applying LOWER() to that result returns 'κόσμοσ' instead
   of the original 'κόσμος'.
   
   According to wikipedia (https://en.wikipedia.org/wiki/Sigma), there are two
   forms of lowercase for Greek capital Sigma (Σ):
   - If Σ is at the end of a word, it should lowercase to ς
   - If Σ is in the middle of a word, it should lowercase to σ
   
   How to repeat:
   ```sql
   SELECT LOWER(UPPER('κόσμος'));
   -- Expected: 'κόσμος'
   -- Actual:   'κόσμοσ'
   ```
   
   

2. Re: BUG #19004: Incorrect lowercasing of word-final Greek capital Sigma (Σ)

   John Naylor <johncnaylorls@gmail.com> — 2025-07-31T07:39:12Z

   On Thu, Jul 31, 2025 at 1:50 PM PG Bug reporting form
   <noreply@postgresql.org> wrote:
   > When applying UPPER() to the Greek word 'κόσμος', PostgreSQL returns
   > 'ΚΌΣΜΟΣ'. However, applying LOWER() to that result returns 'κόσμοσ' instead
   > of the original 'κόσμος'.
   
   Hi,
   
   For built-in collations, context-sensitive case mappings are a
   forthcoming feature which will be available in PG18 later this year,
   with the pg_unicode_fast collation:
   
   CREATE TABLE example (
   a text COLLATE PG_UNICODE_FAST,
   b text COLLATE PG_C_UTF8
   );
   
   INSERT INTO example VALUES ('κόσμος', 'κόσμος');
   
   SELECT LOWER(UPPER(a)), LOWER(UPPER(b)) FROM example;
    lower  | lower
   --------+--------
    κόσμος | κόσμοσ
   (1 row)
   
   -- 
   John Naylor
   Amazon Web Services