perl - Why CONCAT() does not default to default charset in MySQL?

Question

What is the reason, that using CONCAT() in pure UTF-8 environment MySQL still treats concatenated string (when some col in expression is for example int or date) as some other charset (probably Latin-1)?

MySQL environment seen from client (s):

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

Test dataset:

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, '???ü??', 2011);

This query is good:

SELECT id, title FROM utf8_test;

This one turns utf-8 flag off (already in MySQL, AFIU):

SELECT CONCAT(id, title) FROM utf8_test;

From mysql-client everything seems fine, because it is set to show chars as UTF-8, but when running through perl DBI, all results of queries having CONCAT() inside don't have utf-8 flag set. Example code:

#!/usr/bin/perl

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

my $dbh = your_db_connect_routine('test');

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

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

There is at least two workarounds i know

• quering with CAST() SELECT CONCAT( CAST(id AS CHAR CHARACTER SET utf8), title) FROM utf8_test
• using $str = Encode::_utf8_on($str) (is considered as bad practice?)

but i am asking: why it is in MySQL so? Should i consider it as bug or feature?

Answer 1

It's a well known bug in MySQL. It's fixed in MySQL 5.5

See: http://bugs.mysql.com/bug.php?id=12030

The issue stems from concatenating an integer with a varchar.

The work around is to cast the id (integer) first to a char, and then concatenate, ie:

SELECT CONCAT(cast(id as char), title) FROM utf8_test