mysql> explain SELECT a.id FROM a INNER join b on a.id = b.id;
+----+-------------+-----------+--------+---------------------------------+---------+---------+------------------------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+--------+---------------------------------+---------+---------+------------------------+----------+-------------+
| 1 | SIMPLE | b | index | PRIMARY,b_id_index | id2 | 4 | NULL | 88343401 | Using index |
| 1 | SIMPLE | a | eq_ref | PRIMARY,a_id_index | PRIMARY | 92 | db.b.id | 1 | Using index |
+----+-------------+-----------+--------+---------------------------------+---------+---------+------------------------+----------+-------------+
2 rows in set (0.01 sec)

mysql> SELECT VERSION();
+------------+
| VERSION() |
+------------+
| 5.1.73-log |
+------------+
1 row in set (0.00 sec)


CREATE TABLE `a` (
`id` varchar(30) NOT NULL,
`id2` int(12) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `id2` (`id2`),
KEY `a_id_index` (`id`) )ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `b` (
`id` varchar(30) NOT NULL,
`id2` int(12) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `id2` (`id2`),
KEY `b_id_index` (`id`) )ENGINE=InnoDB DEFAULT CHARSET=utf8

机器空闲内存:8.6G