Custom sorting (order by) in PostgreSQL, independent of locale - sql

Question

Let's say I have a simple table with two columns: id (int) and name (varchar). In this table I store some names which are in Polish, e.g.:
1 | sępoleński
2 | świecki
3 | toruński
4 | Włocławek
Now, let's say I want to sort the results by name:
SELECT * FROM table ORDER BY name;
If I have C locale, I get:
4 | Włocławek
1 | sępoleński
3 | toruński
2 | świecki
which is wrong, because "ś" should be after "s" and before "t". If I use Polish locale (pl_PL.UTF-8), I get:
1 | sępoleński
2 | świecki
3 | toruński
4 | Włocławek
which is also not what I want, because I would like names starting with capital letters to be first just like in C locale, like this:
4 | Włocławek
1 | sępoleński
2 | świecki
3 | toruński
How can I do this?

Answer 1

If you want a custom sort, you must define some function that modifies your values in some way so that the natural ordering of the modified values fits your requirement.
For example, you can append some character or string it the value starts with uppercase:
CREATE OR REPLACE FUNCTION mysort(text) returns text IMMUTABLE as $$
SELECT CASE WHEN substring($1 from 1 for 1) =
upper( substring($1 from 1 for 1)) then 'AAAA' || $1 else $1 END
;
$$ LANGUAGE SQL;
And then
SELECT * FROM table ORDER BY mysort(name);
This is not foolprof (you might want to change 'AAA' for something more apt) and hurts performance, of course.

Answer 2

If you want it efficient, you'll need to create another column that "naturally" sorts correctly (e.g. even in the C locale), and use that as a sorting criterion. For that, you should use the approach of the strxfrm C library function. As a straight-forward strxfrm table for your approach, replace each letter with two ASCII letters: 's' would become 's0' and 'ś' would become 's1'. Then 'świecki' becomes 's1w0i0e0c0k0i0', and the regular ASCII sorting will sort it correctly.
If you don't want to create a separate column, you can try to use a function in the where clause:
SELECT * FROM table ORDER BY strxfrm(name);
Here, strxfrm needs to be replaced with a proper function. Either you write one yourself, or you use the standard translate function (although this doesn't support replacing a character with two of them, so you'll need some more involved transformation).