Postgres upper function on turkish character does not return expected result

Question

It looks like postgres upper/lower function does not handle select characters in Turkish character set.

select upper(\'Aaı\'), lower(\'Aaİ\') from          


        

Answer 1

This is indeed bug in PostgreSQL (still not fixed, even in current git tree). Proof: https://github.com/postgres/postgres/blob/master/src/port/pgstrcasecmp.c

PostgreSQL developers even mention specifically those Turkish characters there:

SQL99 specifies Unicode-aware case normalization, which we don't yet have the infrastructure for. Instead we use tolower() to provide a locale-aware translation. However, there are some locales where this is not right either (eg, Turkish may do strange things with 'i' and 'I'). Our current compromise is to use tolower() for characters with the high bit set, and use an ASCII-only downcasing for 7-bit characters.

pg_upper() implemented in this file is extremely simplistic (as its companion pg_tolower()):

unsigned char
pg_toupper(unsigned char ch)
{
    if (ch >= 'a' && ch <= 'z')
            ch += 'A' - 'a';
    else if (IS_HIGHBIT_SET(ch) && islower(ch))
            ch = toupper(ch);
    return ch;
}

As you can see, this code does not treat its parameter as Unicode code point, and cannot possibly work 100% correctly, unless currently selected locale happens to be the one that we care for (like Turkish non-unicode locale) and OS-provided non-unicode toupper() is working correctly.

This is really sad, I just hope that this will be solved in upcoming PostgreSQL releases...