MySQL的JOIN（四）：JOIN优化实践之快速匹配
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。
