I have a table constructed like this :
oid | identifier | value 1 | 10 | 101 2 | 10 | 102 3 | 20 | 201 4 | 20 | 202 5
You have to create an aggregate function, e.g.
CREATE AGGREGATE array_accum (anyelement) ( sfunc = array_append, stype = anyarray, initcond = '{}' );
then
SELECT identifier, array_accum(value) AS values FROM table GROUP BY identifier;
HTH