(select 'head1' "heads", array_agg(distinct head1 order by head1) "values" from flattable) union all 
(select 'head2' "heads", array_agg(distinct head2 order by head2) "values" from flattable) union all 
(select 'head3' "heads", array_agg(distinct head3 order by head3) "values" from flattable)

-- output:
--
--  heads |        values         
-- -------+-----------------------
--  head1 | {val1,val2,val3,val6}
--  head2 | {val2,val4,val7}
--  head3 | {val3,val5}
-- (3 rows)