Very slow join of the small table to the big one - mysql

Question

I needed to add one more column to already existing one (~6M rows).
Unfortunately, I cannot modify the table.
So, I created another one, which contains only two columns: the first (DI_key) corresponds to the primary key of the large table (ID_key) and the second (int(11)) corresponds to the property I want to keep in the database.
For the moment the second table contains only ~250 rows, but when I LEFT JOIN it ON (DI_key = ID_key) my requests to the bigger table are ~ 1000 times slower!
Adding indexes on the small table doesn't solve the problem.
Any suggestions would be welcome...
Unfortunately, I'm working on the very old infrastructure - MySQL 5.1
Technical infos:
--
-- Structure de la table `line`
--
CREATE TABLE IF NOT EXISTS `line` (
`ID_line` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`LISOCI` char(2) DEFAULT '02',
`LICLIE` int(11) DEFAULT '0',
`LIADLIV` tinyint(4) NOT NULL DEFAULT '0',
`LICMD` int(11) NOT NULL DEFAULT '0',
`LICMDM` varchar(32) NOT NULL,
`LILIG` int(11) NOT NULL DEFAULT '0',
`LIARTI` varchar(8) NOT NULL,
`LIDES1` varchar(40) NOT NULL,
`LIDES2` varchar(40) NOT NULL,
`LIQTE1` float DEFAULT '0',
`LIQTE2` float DEFAULT '0',
`LIQTE3` float DEFAULT '0',
`LIQTE4` float DEFAULT '0',
`LIQTE5` float DEFAULT '0',
`LIQTE6` float DEFAULT '0',
`LIQTE7` float DEFAULT '0',
`LIQTE8` float DEFAULT '0',
`LIQTE9` float DEFAULT '0',
`LIQTE10` float DEFAULT '0',
`LIQTE11` float DEFAULT '0',
`LIQTE12` float DEFAULT '0',
`LICOMMENT` varchar(80) NOT NULL,
`LICMT1` varchar(80) NOT NULL,
`LICMT2` varchar(80) NOT NULL,
`LICMT3` varchar(80) NOT NULL,
`LICMT4` varchar(80) NOT NULL,
`LICMT5` varchar(80) NOT NULL,
`LICMT6` varchar(80) NOT NULL,
`LICMT7` varchar(80) NOT NULL,
`LICMT8` varchar(80) NOT NULL,
`LICMT9` varchar(80) NOT NULL,
`LICMT10` varchar(80) NOT NULL,
`LICMT11` varchar(80) NOT NULL,
`LICMT12` varchar(80) NOT NULL,
`LIUNIV` char(3) NOT NULL,
`LILIBQTE` varchar(10) DEFAULT NULL,
`LIQNIF` float NOT NULL DEFAULT '0',
`LIDATE` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`LIPRIX` float NOT NULL DEFAULT '0',
`LIPVCONSO` double NOT NULL DEFAULT '0',
`LITOTAL` float NOT NULL DEFAULT '0',
`LIPRV` double NOT NULL DEFAULT '0',
`LILOGON` varchar(12) NOT NULL,
`LIADRIP` varchar(16) NOT NULL,
`LISESSION` varchar(32) NOT NULL,
`LIGRATUIT` varchar(1) NOT NULL DEFAULT 'N',
PRIMARY KEY (`ID_line`),
KEY `I1` (`LISOCI`,`LICLIE`,`LICMD`,`LILIG`),
KEY `I2` (`LISOCI`,`LICLIE`,`LICMD`,`LISESSION`,`LILIG`),
KEY `L12` (`LICMDM`,`LILIG`),
KEY `LICMD` (`LICMD`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=7185054 ;
--
-- Structure de la table `line_info`
--
CREATE TABLE IF NOT EXISTS `line_info` (
`DI_line` bigint(20) unsigned NOT NULL,
`LICLIES` int(11) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`DI_line`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
SHOW TABLE STATUS WHERE name LIKE "line":
+------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+---------------------+-------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+---------------------+-------------------+----------+----------------+---------+
| line | MyISAM | 10 | Dynamic | 6339101 | 179 | 1135581592 | 281474976710655 | 429571072 | 3356 | 7185054 | 2023-01-27 09:31:29 | 2023-02-06 14:49:08 | 2023-01-27 09:34:49 | latin1_swedish_ci | NULL | | |
+------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+---------------------+-------------------+----------+----------------+---------+
1 row in set (0.00 sec)
SHOW TABLE STATUS WHERE name LIKE "line_info":
+-----------+--------+---------+------------+------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+-----------+--------+---------+------------+------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+---------+
| line_info | MyISAM | 10 | Fixed | 239 | 13 | 3107 | 3659174697238527 | 6144 | 0 | NULL | 2023-02-07 09:50:45 | 2023-02-07 09:50:45 | NULL | latin1_swedish_ci | NULL | | |
+-----------+--------+---------+------------+------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+---------+
1 row in set (0.01 sec)
EXPLAIN SELECT:
EXPLAIN SELECT SQL_NO_CACHE `line`.*, `line_info`.`LICLIES`
FROM `line`
LEFT JOIN `line_info` ON DI_line = ID_line
WHERE (LISOCI LIKE '02')
AND (LICLIE IN ('44', '55') OR LICLIES IN ('44', '55'))
AND LICMD = 0
AND LICMDM = ''
AND LILOGON = 'WWW'
AND LIDATE >= '2022-12-02 16:53:56'
Result:
+----+-------------+-----------+--------+-----------------+---------+---------+--------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+--------+-----------------+---------+---------+--------------+-------+-------------+
| 1 | SIMPLE | line | ref | I1,I2,L12,LICMD | L12 | 34 | const | 56594 | Using where |
| 1 | SIMPLE | line_info | eq_ref | PRIMARY | PRIMARY | 8 | line.ID_line | 1 | Using where |
+----+-------------+-----------+--------+-----------------+---------+---------+--------------+-------+-------------+
2 rows in set (0.00 sec)