I would note that "foo IN (a, b, c)" can often be better done as "foo = ANY(ARRAY[a, b, c])" in Postgres - not least because you can shift to passing in an array of variable size without having to construct a special statement for each.

For sorting on the result of a function, consider using a subselect. "SELECT foo FROM (select fun(z) as foo from blaz) tmp order by 1".