Re: powerset?

Very nice, thanks!

On Sep 23, 2006, at 10:47 PM, Michael Fuhr wrote:

> On Fri, Sep 22, 2006 at 11:38:12PM -0700, Ben wrote:
>> Does anybody have a stored proc they'd like to share (preferably pl/
>> pgsql) that generates the power set of an array?
>
> Here's an attempt:
>
> CREATE FUNCTION powerset(a anyarray) RETURNS SETOF anyarray AS $$
> DECLARE
> retval a%TYPE;
> alower integer := array_lower(a, 1);
> aupper integer := array_upper(a, 1);
> j integer;
> k integer;
> BEGIN
> FOR i IN 0 .. (1 << (aupper - alower + 1)) - 1 LOOP
> retval := '{}';
> j := alower;
> k := i;
>
> WHILE k > 0 LOOP
> IF k & 1 = 1 THEN
> retval := array_append(retval, a[j]);
> END IF;
>
> j := j + 1;
> k := k >> 1;
> END LOOP;
>
> RETURN NEXT retval;
> END LOOP;
>
> RETURN;
> END;
> $$ LANGUAGE plpgsql IMMUTABLE STRICT;
>
> Since this is a set-returning function you'd call it as follows:
>
> SELECT * FROM powerset(ARRAY[1,2,3]);
> powerset
> ----------
> {}
> {1}
> {2}
> {1,2}
> {3}
> {1,3}
> {2,3}
> {1,2,3}
> (8 rows)
>
> If you wrap the PL/pgSQL function with an SQL function then you
> could call it from the SELECT list:
>
> CREATE FUNCTION powerset2(anyarray) RETURNS SETOF anyarray AS $$
> SELECT * FROM powerset($1);
> $$ LANGUAGE sql IMMUTABLE STRICT;
>
> CREATE TABLE foo (id serial PRIMARY KEY, a integer[]);
> INSERT INTO foo (a) VALUES ('{1,2}');
> INSERT INTO foo (a) VALUES ('{10,20,30}');
>
> SELECT id, powerset2(a) FROM foo;
> id | powerset2
> ----+------------
> 1 | {}
> 1 | {1}
> 1 | {2}
> 1 | {1,2}
> 2 | {}
> 2 | {10}
> 2 | {20}
> 2 | {10,20}
> 2 | {30}
> 2 | {10,30}
> 2 | {20,30}
> 2 | {10,20,30}
> (12 rows)
>
> Will that work for you?
>
> --
> Michael Fuhr