Unexpected function behaviour with NULL and/or default NULL parameters

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Hello,

I'm experiencing odd behaviour with a function I wrote yesterday.

Background: function is supposed to deliver some "terms and
conditions" from a table; when the "locale" is found, deliver the
highest version of that, otherwise, deliver the highest version of the
"default" locale.

CREATE OR REPLACE FUNCTION
public.get_current_tac(userid bigint, sessionid uuid, locale character
varying, OUT current_tac json)
RETURNS json
LANGUAGE sql
IMMUTABLE STRICT SECURITY DEFINER
AS $function$
SELECT json_agg(selected) FROM (
SELECT * FROM (
SELECT *, 1 AS locale_specific FROM terms_and_conditions WHERE
locale = $3 ORDER BY version DESC LIMIT 1
) specific
UNION
SELECT * FROM (
SELECT *, 0 AS locale_specific FROM terms_and_conditions WHERE
locale = 'default' ORDER BY version DESC LIMIT 1
) unspecific
ORDER BY locale_specific DESC
LIMIT 1
) selected;
$function$

This works fine when I give valid userid, sessionid and locale:
=# select
get_current_tac(userid:=38,sessionid:='79993643-ec3c-0359-f603-069b543ce4a8'::uuid,locale:='en');

get_current_tac
-
----------------------------------------------------------------------------------------------------------------------------------------------------------
[{"locale":"default","version":1,"updated_at":"2015-05-05T14:04:17.246684+02:00","terms_and_conditions":"Hier kommen die AGB rein","locale_specific":0}]
(1 row)

Then I realised I don't really need those first two parameters and
applied default values (NULL). As $1 and $2 are not used, it should
still work, right? Well, it returns one empty (!) row (behaviour is
the same when declaring NULL as default values in the function header,
can't show as there's another version with only "locale" as parameter):

# select get_current_tac(userid:=null, sessionid:=null::uuid,
locale:='en');
get_current_tac
- ------------------------
<NULL>
(1 row)

I'm completely puzzled by this behaviour; at least it should cast the
"locale_specific" value into the JSON output, shouldn't it?

What am I missing? Any hints appreciated!
- --
Gunnar "Nick" Bluth
RHCE/SCLA

Mobil +49 172 8853339
Email: gunnar(dot)bluth(at)pro-open(dot)de
_____________________________________________________________
In 1984 mainstream users were choosing VMS over UNIX.
Ten years later they are choosing Windows over UNIX.
What part of that message aren't you getting? - Tom Payne

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