I think when you call a function with a NULL param, pl/pgsql treats
the param as type 'unknown' and matches it to any type in the
function's parameter list. The problem comes when you have so many
NULLs that more than 1 function would match. In that case, you can
cast your NULL so that the proper function is called.
Example:
create function vk_test(integer,varchar)
returns integer as '
begin
raise notice ''integer first function'';
return 0;
end;' language 'plpgsql';
create function vk_test(varchar,integer)
returns integer as '
begin
raise notice ''varchar first function'';
return 0;
end;' language 'plpgsql';
# select vk_test(1, NULL);
NOTICE: integer first function
# select vk_test(NULL, 1);
NOTICE: varchar first function
# select vk_test('a'::varchar, NULL);
NOTICE: varchar first function
# select vk_test(NULL, 'a'::varchar);
NOTICE: integer first function
# select vk_test(NULL, NULL);
ERROR: Function 'vk_test(unknown,unknown)' does not exist
# select vk_test(NULL::integer, NULL::varchar);
NOTICE: integer function
Using explicit casting of nulls allows you to choose the
function with the signature that you want.
