mysql中的快连接_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 TABLEt1 (

idINT PRIMARY KEYAUTO_INCREMENT,

typeINT);SELECT COUNT(*) FROMt1;+----------+

| COUNT(*) |

+----------+

| 110000 |

+----------+

CREATE TABLEt2 (

idINT PRIMARY KEYAUTO_INCREMENT,

typeINT);SELECT COUNT(*) FROMt2;+----------+

| 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 ONt1(type);

EXPLAINSELECT * 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 ONt1;CREATE INDEX idx_type ONt2(type);

EXPLAINSELECT * 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 ONt2;--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 ONt2(type);

EXPLAINSELECT * 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 ONt2;CREATE INDEX idx_type ONt1(type);

EXPLAINSELECT * 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。

DROP INDEX idx_type ONt1;--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 ONt2(type);

EXPLAINSELECT * 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 ONt2;CREATE INDEX idx_type ONt1(type);

EXPLAINSELECT * 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。