You could write a set of expressions that yield proper order by field
in one case and say null in another. Say we want order by columns
<col1, col2 desc> if the first function argument is '1' and by <col3
desc, col1, col2> if it is '2', this can be achieved as:
select col1, col2, col3 from mytable
where ...
order by case when $1 = '1' then col1 end,
case when $1 = '1' then col2 else col3 end desc,
case when $1 = '2' then col1 end,
case when $1 = '2' then col2 end
;
This would work as following when $1 = '1':
select col1, col2, col3 from mytable
where ...
order by col1, col2desc, null, null;
and when $1 = '2':
select col1, col2, col3 from mytable
where ...
order by null, col3 desc, col1, col2
