MySQL多表连接过程中自动创建索引一例:

一 创建表如下 CREATE TABLE t1 (id INT PRIMARY KEY, a INT UNIQUE,
b INT, c INT, e INT, KEY bc (b, c));
INSERT INTO t1 VALUES (0, 0, NULL, 0, 0), (1, 1, 1, 1, 1), (2, 2, 1, 2, 2),
(3, 3, 1, 1, 3), (6, 6, 1, 1, 6),
(7, 7, 2, 2, 7), (8, 8, 3, 3, 8), (9, 9, 4, 5, NULL);
CREATE TABLE t2 (id INT PRIMARY KEY, a INT UNIQUE,
b INT, c INT, e INT, KEY bc (b, c));
INSERT INTO t2 VALUES (0, 0, NULL, 0, 0), (1, 1, 1, 1, 1), (2, 2, 1, 2, 2),
(3, 3, 1, 1, 3), (4, 4, 2, 1, 4), (5, 5, 2, 2, 5), (6, 6, 1, 1, 6),
(7, 7, 2, 2, 7), (8, 8, 3, 3, 8), (9, 9, 4, 5, NULL);
CREATE TABLE t3 (id INT PRIMARY KEY, a INT UNIQUE,
b INT, c INT, e INT, KEY bc (b, c));
INSERT INTO t3 VALUES (0, 0, NULL, 0, 0), (1, 1, 1, 1, 1), (2, 2, 1, 2, 2),
(3, 3, 1, 1, 3), (4, 4, 2, 1, 4), (5, 5, 2, 2, 5), (6, 6, 1, 1, 6),
(8, 8, 3, 3, 8), (9, 9, 4, 5, NULL);

二 执行查询如下 mysql>  explain SELECT * FROM t1 WHERE e IN (SELECT e FROM t2 WHERE c IN (SELECT c FROM t3));
+----+--------------+-------------+------------+--------+---------------+------------+---------+-----------+------+----------+------------------------
| id | select_type  | table       | partitions | type   | possible_keys | key        | key_len | ref       | rows | filtered | Extra
+----+--------------+-------------+------------+--------+---------------+------------+---------+-----------+------+----------+------------------------
|  1 | SIMPLE       | t1          | NULL       | ALL    | NULL          | NULL       | NULL    | NULL      |    8 |   100.00 | Using where
|  1 | SIMPLE       | <subquery2> | NULL       | eq_ref | <auto_key>    | <auto_key> | 5       | test.t1.e |    1 |   100.00 | NULL
|  2 | MATERIALIZED | t3          | NULL       | index  | NULL          | bc         | 10      | NULL      |    9 |   100.00 | Using index
|  2 | MATERIALIZED | t2          | NULL       | ALL    | NULL          | NULL       | NULL    | NULL      |   10 |    10.00 | Using where; Using join buffer (Block Nested Loop) |
+----+--------------+-------------+------------+--------+---------------+------------+---------+-----------+------+----------+------------------------
4 rows in set, 1 warning (0.10 sec)三 分析
1 t2和t3被物化，这是一个优化点
2 物化了的t2和t3做了一个join操作，结果是一个子查询‘<subquery2>’的一部分
3 对于<subquery2>子查询，“type”列的值，是eq_ref, 通常主键才有此值（唯一键只能用到ref）。 这是因为MySQL提供了另外的优化点，即：
  对于物化的对象，可以为其自动创建索引，这表现在了“possible_keys” 的值为 ‘<auto_key>’上。