Array order with postgres

array_order.sql

CREATE OR REPLACE FUNCTION idx(anyarray, anyelement)
  RETURNS int AS 
$$
  SELECT i FROM (
     SELECT generate_series(array_lower($1,1),array_upper($1,1))
  ) g(i)
  WHERE $1[i] = $2
  LIMIT 1;
$$ LANGUAGE sql IMMUTABLE;

gistfile1.txt

test=# create table foo (bar int);
CREATE TABLE

test=# insert into foo (bar) values (1), (2), (3), (4);
INSERT 0 4

test=# select * from foo;
 bar 
-----
   1
   2
   3
   4
(4 rows)

test=# select * from foo where foo.bar in (4,1,3,2);
 bar 
-----
   1
   2
   3
   4
(4 rows)

test=# select * from foo order by idx(array[4,1,3,2], bar);
 bar 
-----
   4
   1
   3
   2
(4 rows)

test=# select * from foo where bar in (4,1) order by idx(array[4,1,3,2], bar);
 bar 
-----
   4
   1