这篇博文讲述如何优化扫描速度。我们通过MySQL的JOIN（二）：JOIN原理得知了两张表的JOIN操作就是不断从驱动表中取出记录，然后查找出被驱动表中与之匹配的记录并连接。这个过程的实质就是查询操作，想要优化查询操作，建索引是最常用的方式。那索引怎么建呢？我们来讨论下，首先插入测试数据。
CREATE TABLE t1 (
id INT PRIMARY KEY AUTO_INCREMENT,
type INT
);
SELECT COUNT(*) FROM t1;
+----------+
| COUNT(*) |
+----------+
| 110000 |
+----------+
CREATE TABLE t2 (
id INT PRIMARY KEY AUTO_INCREMENT,
type INT
);
SELECT COUNT(*) FROM t2;
+----------+
| COUNT(*) |
+----------+
| 100 |
+----------+
左连接
左连接中，左表是驱动表，右表是被驱动表。想要快速查找被驱动表中匹配的记录，所以我们可以在右表建索引，从而提高连接性能。
-- 首先两个表都没建索引
EXPLAIN SELECT * FROM t1 LEFT JOIN t2 ON t1.type=t2.type;
+----+-------+------+------+--------+----------------------------------------------------+
| id | table | type | key | rows | Extra |
+----+-------+------+------+--------+----------------------------------------------------+
| 1 | t1 | ALL | NULL | 110428 | NULL |
| 1 | t2 | ALL | NULL | 100 | Using where; Using join buffer (Block Nested Loop) |
+----+-------+------+------+--------+----------------------------------------------------+
-- 尝试在左表建立索引，改进不大
CREATE INDEX idx_type ON t1(type);
EXPLAIN SELECT * FROM t1 LEFT JOIN t2 ON t1.type=t2.type;
+----+-------+-------+----------+--------+----------------------------------------------------+
| id | table | type | key | rows | Extra |
+----+-------+-------+----------+--------+----------------------------------------------------+
| 1 | t1 | index | idx_type | 110428 | Using index |
| 1 | t2 | ALL | NULL | 100 | Using where; Using join buffer (Block Nested Loop) |
+----+-------+-------+----------+--------+----------------------------------------------------+ -- 尝试在右表建立索引，效果拔群，Using index!!!
DROP INDEX idx_type ON t1;
CREATE INDEX idx_type ON t2(type);
EXPLAIN SELECT * FROM t1 LEFT JOIN t2 ON t1.type=t2.type;
+----+-------+------+---------------+----------+--------+-------------+
| id | table | type | possible_keys | key | rows | Extra |
+----+-------+------+---------------+----------+--------+-------------+
| 1 | t1 | ALL | NULL | NULL | 110428 | NULL |
| 1 | t2 | ref | idx_type | idx_type | 1 | Using index |
+----+-------+------+---------------+----------+--------+-------------+
右连接
右连接中，右表是驱动表，左表是被驱动表，想要快速查找被驱动表中匹配的记录，所以我们可以在左表建索引，从而提高连接性能。
DROP INDEX idx_type ON t2;
-- 两个表都没有索引
EXPLAIN SELECT * FROM t1 RIGHT JOIN t2 ON t1.type=t2.type;
+----+-------+------+------+--------+----------------------------------------------------+
| id | table | type | key | rows | Extra |
+----+-------+------+------+--------+----------------------------------------------------+
| 1 | t2 | ALL | NULL | 100 | NULL |
| 1 | t1 | ALL | NULL | 110428 | Using where; Using join buffer (Block Nested Loop) |
+----+-------+------+------+--------+----------------------------------------------------+ -- 在右边建立索引，改进不大
CREATE INDEX idx_type ON t2(type);
EXPLAIN SELECT * FROM t1 RIGHT JOIN t2 ON t1.type=t2.type;
+----+-------+-------+---------------+----------+--------+----------------------------------------------------+
| id | table | type | possible_keys | key | rows | Extra |
+----+-------+-------+---------------+----------+--------+----------------------------------------------------+
| 1 | t2 | index | NULL | idx_type | 100 | Using index |
| 1 | t1 | ALL | NULL | NULL | 110428 | Using where; Using join buffer (Block Nested Loop) |
+----+-------+-------+---------------+----------+--------+----------------------------------------------------+ -- 尝试在左边建立索引，效果拔群！
DROP INDEX idx_type ON t2;
CREATE INDEX idx_type ON t1(type);
EXPLAIN SELECT * FROM t1 RIGHT JOIN t2 ON t1.type=t2.type;
+----+-------+------+---------------+--------------+------+-------------+
| id | table | type | possible_keys | ref | rows | Extra |
+----+-------+------+---------------+--------------+------+-------------+
| 1 | t2 | ALL | NULL | NULL | 100 | NULL |
| 1 | t1 | ref | idx_type | test.t2.type | 5 | Using index |
+----+-------+------+---------------+--------------+------+-------------+
内连接
我们知道，MySQL Optimizer会对内连接做优化，不管谁内连接谁，都是用小表驱动大表，所以如果要优化内连接，可以在大表上建立索引，以提高连接性能。
另外注意一点，在小表上建立索引时，MySQL Optimizer会认为用大表驱动小表效率更快，转而用大表驱动小表。
对内连接小表驱动大表的优化策略不清楚的话，可以看MySQL的JOIN（三）：JOIN优化实践之内循环的次数
DROP INDEX idx_type ON t1;
-- 两个表都没有索引，t2驱动t1
EXPLAIN SELECT * FROM t1 INNER JOIN t2 ON t1.type=t2.type;
+----+-------+------+------+--------+----------------------------------------------------+
| id | table | type | key | rows | Extra |
+----+-------+------+------+--------+----------------------------------------------------+
| 1 | t2 | ALL | NULL | 100 | NULL |
| 1 | t1 | ALL | NULL | 110428 | Using where; Using join buffer (Block Nested Loop) |
+----+-------+------+------+--------+----------------------------------------------------+
-- 在t2表上建立索引，MySQL的Optimizer发现后，用大表驱动了小表
CREATE INDEX idx_type ON t2(type);
EXPLAIN SELECT * FROM t1 INNER JOIN t2 ON t1.type=t2.type;
+----+-------+------+----------+--------+-------------+
| id | table | type | key | rows | Extra |
+----+-------+------+----------+--------+-------------+
| 1 | t1 | ALL | NULL | 110428 | Using where |
| 1 | t2 | ref | idx_type | 1 | Using index |
+----+-------+------+----------+--------+-------------+ -- 在t1表上建立索引，再加上t1是大表，符合“小表驱动大表”的原则，性能比上面的语句要好
DROP INDEX idx_type ON t2;
CREATE INDEX idx_type ON t1(type);
EXPLAIN SELECT * FROM t1 INNER JOIN t2 ON t1.type=t2.type;
+----+-------+------+---------------+----------+------+-------------+
| id | table | type | possible_keys | key | rows | Extra |
+----+-------+------+---------------+----------+------+-------------+
| 1 | t2 | ALL | NULL | NULL | 100 | Using where |
| 1 | t1 | ref | idx_type | idx_type | 5 | Using index |
+----+-------+------+---------------+----------+------+-------------+
三表连接
上面都是两表连接，三表连接也是一样的，找出驱动表和被驱动表，在被驱动表上建立索引，即可提高连接性能。
总结
想要从快速匹配的角度优化JOIN，首先就是找出谁是驱动表，谁是被驱动表，然后在被驱动表上建立索引即可。