前文说了EXPLAIN的输出的含义，本文实战一下。
Database Schema
DROP DATABASE dbTest;
CREATE DATABASE dbTest;
USE dbTest;
CREATE TABLE t1
(
c_primary_key INT,
c_unique_key CHAR(64),
c_unique_not_null_key CHAR(64) NOT NULL,
c_key CHAR(64),
c_multi_key_part1 CHAR(64),
c_multi_key_part2 CHAR(64),
c_int_value INT,
c_str_value CHAR(64),
PRIMARY KEY(c_primary_key),
UNIQUE KEY(c_unique_key),
UNIQUE KEY(c_unique_not_null_key),
KEY(c_multi_key_part1, c_multi_key_part2),
KEY(c_key)
)ENGINE=InnoDB;
CREATE TABLE t2
(
c_primary_key INT,
c_unique_key CHAR(64),
c_unique_not_null_key CHAR(64) NOT NULL,
c_key CHAR(64),
c_multi_key_part1 CHAR(64),
c_multi_key_part2 CHAR(64),
c_int_value INT,
c_str_value CHAR(64),
c_t1_primary_key INT,
PRIMARY KEY(c_primary_key),
UNIQUE KEY(c_unique_key),
UNIQUE KEY(c_unique_not_null_key),
KEY(c_multi_key_part1, c_multi_key_part2),
KEY(c_key),
UNIQUE KEY(c_t1_primary_key)
)ENGINE=InnoDB;
Join类型
const
-
使用primary key查找一条记录，满足const条件。
mysql> EXPLAIN SELECT * FROM dbTest.t1 WHERE c_primary_key=1; +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | t1 | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ -
使用unique key查找一条非NULL记录，满足const条件。
mysql> EXPLAIN SELECT * FROM dbTest.t1 WHERE c_unique_key='4cb15758c8e311e5b46f06af68695f49'; +----+-------------+-------+-------+---------------+--------------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+--------------+---------+-------+------+-------+ | 1 | SIMPLE | t1 | const | c_unique_key | c_unique_key | 65 | const | 1 | NULL | +----+-------------+-------+-------+---------------+--------------+---------+-------+------+-------+ -
使用unique key查找NULL记录，这种情况下NULL记录可能有多条，所以不满足const条件。而是ref条件，ref意味着可能得到匹配的结果不唯一，即可能存在多条。那么对于unique key，可以为NULL，那么我们再来看：
mysql> EXPLAIN SELECT * FROM dbTest.t1 WHERE c_unique_key is NULL; +----+-------------+-------+------+---------------+--------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+--------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | t1 | ref | c_unique_key | c_unique_key | 65 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+--------------+---------+-------+------+-----------------------+ -
使用非NULL的unique key来查询，跟primary key类似，都是唯一的，所以满足const条件。
mysql> EXPLAIN SELECT * FROM dbTest.t1 WHERE c_unique_not_null_key='00047412c96511e5844906af68695f49' limit 1; +----+-------------+-------+-------+-----------------------+-----------------------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+-----------------------+-----------------------+---------+-------+------+-------+ | 1 | SIMPLE | t1 | const | c_unique_not_null_key | c_unique_not_null_key | 64 | const | 1 | NULL | +----+-------------+-------+-------+-----------------------+-----------------------+---------+-------+------+-------+ -
使用非唯一的index列查询，可能存在多条记录，所以是ref而不是const。
mysql> EXPLAIN SELECT * FROM dbTest.t1 WHERE c_key='4cb15758c8e311e5b46f06af68695f49'; +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------+ | 1 | SIMPLE | t1 | ref | c_key | c_key | 65 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------+const至多有一条记录满足条件！
eq_ref
前文说，eq_ref类型对于之前表的每一个行组合，只从该表中读取一条记录。只有一条记录匹配要求，索引必须是primary key或者unique key（非NULL）。
-
使用primary key进行表关联
mysql> EXPLAIN SELECT * FROM dbTest.t1,dbTest.t2 WHERE t1.c_primary_key=t2.c_key; +----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+ | 1 | SIMPLE | t2 | ALL | c_key | NULL | NULL | NULL | 8042 | Using where | | 1 | SIMPLE | t1 | eq_ref | PRIMARY | PRIMARY | 4 | dbTest.t2.c_key | 1 | Using where | +----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+可以看到，对于t2表中的每一行，t1中都有唯一的一行（至多一行）进行匹配，所以最终匹配为eq_ref。
-
使用NOT NULL的unique key（唯一的一行）进行关联
mysql> EXPLAIN SELECT * FROM dbTest.t1,dbTest.t2 WHERE t1.c_unique_not_null_key=t2.c_unique_not_null_key; +----+-------------+-------+--------+-----------------------+-----------------------+---------+---------------------------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+-----------------------+-----------------------+---------+---------------------------------+------+-------+ | 1 | SIMPLE | t1 | ALL | c_unique_not_null_key | NULL | NULL | NULL | 9307 | NULL | | 1 | SIMPLE | t2 | eq_ref | c_unique_not_null_key | c_unique_not_null_key | 64 | dbTest.t1.c_unique_not_null_key | 1 | NULL | +----+-------------+-------+--------+-----------------------+-----------------------+---------+---------------------------------+------+-------+eq_ref至多有一条记录满足条件！
ref
对于之前表的每一个组合，匹配到索引值的所有记录将被读取。例如匹配那些左侧前缀的key（multi-part key），或者非primary key，或者非unique index（匹配值不是NULL），或者unique index（但是匹配值是NULL）。
-
使用index列匹配多行记录
mysql> EXPLAIN SELECT * FROM t1 WHERE t1.c_key='58a95fbac96511e5844906'; +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------+ | 1 | SIMPLE | t1 | ref | c_key | c_key | 65 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------+ -
多表关联时使用index列匹配多行记录
mysql> EXPLAIN SELECT * FROM t1,t2 WHERE t1.c_unique_key=t2.c_key; +----+-------------+-------+------+---------------+-------+---------+------------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+------------------------+------+-------------+ | 1 | SIMPLE | t1 | ALL | c_unique_key | NULL | NULL | NULL | 9307 | Using where | | 1 | SIMPLE | t2 | ref | c_key | c_key | 65 | dbTest.t1.c_unique_key | 1 | NULL | +----+-------------+-------+------+---------------+-------+---------+------------------------+------+-------------+ -
匹配左侧前缀（t1.c_multi_key_part1）的多行记录
mysql> EXPLAIN SELECT * FROM t1,t2 WHERE t1.c_multi_key_part1=t2.c_key; +----+-------------+-------+------+-------------------+-------------------+---------+-----------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+-------------------+-------------------+---------+-----------------+------+-------------+ | 1 | SIMPLE | t2 | ALL | c_key | NULL | NULL | NULL | 4904 | Using where | | 1 | SIMPLE | t1 | ref | c_multi_key_part1 | c_multi_key_part1 | 65 | dbTest.t2.c_key | 1 | NULL | +----+-------------+-------+------+-------------------+-------------------+---------+-----------------+------+-------------+ -
使用unique index查找NULL的记录
mysql> EXPLAIN SELECT * FROM t1 WHERE t1.c_unique_key is NULL; +----+-------------+-------+------+---------------+--------------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+--------------+---------+-------+------+-----------------------+ | 1 | SIMPLE | t1 | ref | c_unique_key | c_unique_key | 65 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+--------------+---------+-------+------+-----------------------+ref可以匹配多行记录！
ref_or_null
从这个关键词可以看出，ref或者NULL，既在ref的基础上加上NULL的搜索。以下例子对应于ref的记录。
-
使用index列匹配多行记录，或者NULL记录
mysql> EXPLAIN SELECT * FROM t1 WHERE t1.c_key='58a95fbac96511e5844906' or t1.c_key is NULL; +----+-------------+-------+-------------+---------------+-------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------------+---------------+-------+---------+-------+------+-----------------------+ | 1 | SIMPLE | t1 | ref_or_null | c_key | c_key | 65 | const | 2 | Using index condition | +----+-------------+-------+-------------+---------------+-------+---------+-------+------+-----------------------+ -
其余不再给输出，参照ref，SQL如下。
mysql> EXPLAIN SELECT * FROM t1,t2 WHERE t1.c_unique_key=t2.c_key or t1.c_unique_key is NULL; mysql> EXPLAIN SELECT * FROM t1,t2 WHERE t1.c_multi_key_part1=t2.c_key or t1.c_multi_key_part1 is NULL;ref_or_null = ref + NULL记录，所以是多行
range
对于一个给定的range，使用index来获取记录。range可以使用=，<>，>，>=，BETWEEN，IN()等操作符。
-
BETWEEN操作符
mysql> EXPLAIN SELECT * FROM t1 WHERE t1.c_primary_key BETWEEN 10 AND 20; +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 10 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ -
IN操作符
mysql> EXPLAIN SELECT * FROM t1 WHERE t1.c_primary_key IN (10, 20); +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
等等，不再赘述。
range使用index，多条记录！
index
该类型跟ALL类型，但是不同之处在于搜索的是index数据，因为index数据比较小，所以效率肯定比ALL要高。分为两种情况：
-
索引数据足够满足要求，即索引数据包括了查询的所有数据（在InnoDB下，索引数据数据存储的内容，包括 索引列+主键列，如下，因为c_key索引数据包括了c_key列值+c_primary_key列值，所以只需遍历索引即可）
mysql> EXPLAIN SELECT c_primary_key,c_key FROM t1; +----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | c_key | 65 | NULL | 4915 | Using index | +----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+ -
需要根据某个索引列的顺序进行查询，如下。第一条EXPLAIN从t1中select出所有的c_primary_key，这是不需要用到c_key（当然用c_key是也可以的）。但是第二条EXPLAIN由于需要按照c_key的进行排序（而c_key index就是有序的），所以只需要c_key index存储的顺序读取出来即可达到排序的功能，所以使用c_key就起到了排序的作用。
mysql> EXPLAIN SELECT c_primary_key FROM t1; +----+-------------+-------+-------+---------------+-----------------------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+-----------------------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | c_unique_not_null_key | 64 | NULL | 4915 | Using index | +----+-------------+-------+-------+---------------+-----------------------+---------+------+------+-------------+ mysql> EXPLAIN SELECT c_primary_key FROM t1 order by c_key; +----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | c_key | 65 | NULL | 4915 | Using index | +----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+index这种情况也就比ALL要好一点，这种SQL需要重点review，以防带来灾难！
ALL
终于到了最差的情况，全表扫描。即没有合适的索引数据，所以只能一行一行的扫描数据了，沦落至此，可想而知效果极差。例如：
-
查询符合条件的记录，如下，由于c_str_value列没有索引，导致只能进行全表扫描。优化方法：可以在c_str_value列上加上索引。
mysql> EXPLAIN SELECT * FROM t1 where t1.c_str_value='1111'; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 4915 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ -
查询一个表的所有记录，如下。优化方法：在满足业务需求的情况下，把查询的所有列改成某几列，这样若是某个索引数据满足条件的话，可以不用遍历全表，而仅仅遍历索引数据即可。
mysql> EXPLAIN SELECT * FROM t1; +----+-------------+-------+------+---------------+------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------+ | 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 4915 | NULL | +----+-------------+-------+------+---------------+------+---------+------+------+-------+最慢的查询，必须得review这些SQL，数据量大的情况下，必然带来灾难！
总结
通过以上，我们可以看到效率排序为： const < eq_ref < ref < ref_or_null(range) < index < ALL，通常index和ALL是需要重点注意的。让我们的嗅觉灵敏起来吧。:)
本着理论指导实践的原则，以上用实例对理论做了实践，难免出错，敬请指正。
遗留问题：
- unique index如何保存NULL的索引？这个key允许多条NULL记录存在么？
- primary index可以为NULL么？
- index列中如何存储NULL数据？