为什么CONCAT（）在MySQL中不默认为默认字符集？

\\s

Server characterset:    utf8
Db     characterset:    utf8
Client characterset:    utf8
Conn.  characterset:    utf8

CREATE TABLE `utf8_test` ( 
    `id` int(10) unsigned NOT NULL auto_increment, 
    `title` varchar(50) collate utf8_estonian_ci default NULL,
    `year` smallint(4) unsigned NOT NULL default \'0\', 
    PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_estonian_ci;

INSERT INTO utf8_test VALUES (1, \'Õäöüšž\', 2011);

SELECT id, title FROM utf8_test;

SELECT CONCAT(id, title) FROM utf8_test;

#!/usr/bin/perl

use strict;
use utf8::all;
use Encode qw(is_utf8);

my $dbh = your_db_connect_routine(\'test\');

my $str = $dbh->selectrow_array(\'SELECT CONCAT(id, title) FROM utf8_test\');
print \"CONCAT: False\\n\" unless ( is_utf8($str) );

my $str = $dbh->selectrow_array(\'SELECT title FROM utf8_test\');
print \"NO CONCAT: False\\n\" unless ( is_utf8($str) );

SELECT CONCAT( CAST(id AS CHAR CHARACTER SET utf8), title) FROM utf8_test

$str = Encode::_utf8_on($str)