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

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