my $rs = $schema->resultset('Thread')->search(undef,
    {
        join => 'posts',
        select => [qw/subject timestamp/],
        '+select' => \'MAX(timestamp) as tmax',
        '+as' => [qw/tmax/],
        order_by => 'tmax DESC',
        group_by => 'me.thread_id',
    },
);

foreach my $x ( $rs->all() ) {
    say $x->get_column('tmax') . " " . $x->subject();
}