aggregate that returns array

Hi,

Has anyone considered creating an aggregate function that returns an
array of all matching rows? I am not sure if this makes much sense from
a speed point of view or is possible, but it would help denormalizing
tables when necessary. For example, consider a table that looks as follows:

SELECT * FROM t;
id | value
----+-------
1 | 1.5
1 | 2.5
1 | 3.5
2 | 4.5
2 | 5.5
(5 rows)

It would be nice to be able to do a query as follows:

SELECT id, agg_array(value) FROM t GROUP BY id;
id | agg_array
----+-----
1 | {1.5,2.5,3.5}
2 | {4.5,5.5}
(2 rows)

Thanks,
David Kaplan