本文来自我的github pages博客http://galengao.github.io/ 即www.gaohuirong.cn
摘要:
- 本篇是根据官网中的每个一点来翻译、举例、验证的；英语不好，所以有些话语未必准确，请自行查看官网，若有些点下面没有例子的是因为当时一下子没有想出那么多来，如果大家有遇上好的例子，欢迎在下面留言我持续更新
- 查看执行计划的关键EXPLAIN
- 版本MYSQL5.6，用到的库是官网例子sakila，自行下载导入
由于要把每个点都翻译出来，还需要举例，所以需要一定的时间，本人先把架构理出来，然后逐个点开始
官网地址：http://dev.mysql.com/doc/refman/5.6/en/explain-output.html
EXPLAIN语句返回MYSLQ的执行计划，通过他返回的信息，我们能了解到MYSQL优化器是如何执行SQL语句的，通过分析他能帮助你提供优化的思路。
语法
MYSQL 5.6.3以前只能EXPLAIN SELECT; MYSQL5.6.3以后就可以EXPLAIN SELECT,UPDATE,DELETE
- EXPLAIN 语法例子：
mysql> explain select customer_id,a.store_id,first_name,last_name, b.manager_staff_id from customer a left join store b on a.store_id=b.store_id;
+----+-------------+-------+--------+---------------+---------+---------+-------------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-------------------+------+-------+
| 1 | SIMPLE | a | ALL | NULL | NULL | NULL | NULL | 599 | NULL |
| 1 | SIMPLE | b | eq_ref | PRIMARY | PRIMARY | 1 | sakila.a.store_id | 1 | NULL |
+----+-------------+-------+--------+---------------+---------+---------+-------------------+------+-------+
2 rows in set- EXPLAIN还有一种语法，类似于desc
mysql> explain actor;
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| actor_id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| first_name | varchar(45) | NO | | NULL | |
| last_name | varchar(45) | NO | MUL | NULL | |
| last_update | timestamp | NO | | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
4 rows in setEXPLAIN的输出
EXPLAIN主要包含以下信息：
| Column | JSON Name | Meaning |
|---|---|---|
| id | select_id | The SELECT identifier |
| select_type | None | The SELECT type |
| table | table_name | The table for the output row |
| partitions | partitions | The matching partitions |
| type | access_type | The join type |
| possible_keys | possible_keys | The possible indexes to choose |
| key | key | The index actually chosen |
| key_len | key_length | The length of the chosen key |
| ref | ref | The columns compared to the index |
| rows | rows | Estimate of rows to be examined |
| filtered | filtered | Percentage of rows filtered by table condition |
| Extra | None | Additional information |
id (JSON name: select_id)
SQL查询中的序列号。
select_type (JSON name: none)
查询的类型，可以是下表的任何一种类型：
| select_type Value | JSON Name | Meaning |
|---|---|---|
| SIMPLE | None | 简单查询(不适用union和子查询的) |
| PRIMARY | None | 最外层的查询 |
| UNION | None | UNION中的第二个或者后面的SELECT语句 |
| DEPENDENT UNION | dependent (true) | UNION中的第二个或者后面的SELECT语句,依赖于外部查询 |
| UNION RESULT | union_result | UNION结果 |
| SUBQUERY | None | 子查询中的第一个SELECT语句 |
| DEPENDENT SUBQUERY | dependent (true) | 子查询中的第一个SELECT语句，依赖于外部查询 |
| DERIVED | None | 派生表的SELECT(FROM子句的子查询) |
| MATERIALIZED | materialized_from_subquery | 物化子查询 |
| UNCACHEABLE SUBQUERY | cacheable (false) | 对于该结果不能被缓存，必须重新评估外部查询的每一行子查询 |
| UNCACHEABLE UNION | cacheable (false) | UNION中的第二个或者后面的SELECT语句属于不可缓存子查询 (see UNCACHEABLE SUBQUERY) |
查询类型例子：
1、SIMPLE 简单查询(不适用union和子查询的)
mysql> explain select * from staff;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | staff | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set2、PRIMARY 最外层的查询
mysql> explain select * from (select last_name,first_name from customer) a;
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 599 | NULL |
| 2 | DERIVED | customer | ALL | NULL | NULL | NULL | NULL | 599 | NULL |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
2 rows in set3、UNION UNION中的第二个或者后面的SELECT语句
mysql> explain select first_name,last_name from customer a where customer_id=1 union select first_name,last_name from customer b where customer_id=2;
+------+--------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+--------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
| 1 | PRIMARY | a | const | PRIMARY | PRIMARY | 2 | const | 1 | NULL |
| 2 | UNION | b | const | PRIMARY | PRIMARY | 2 | const | 1 | NULL |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+------+--------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
3 rows in set4、DEPENDENT UNION UNION中的第二个或者后面的SELECT语句，依赖于外部查询
mysql> explain select * from customer where customer_id in(select customer_id from customer a where customer_id=1 union all select customer_id from customer b where customer_id=2);
+------+--------------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+--------------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
| 1 | PRIMARY | customer | ALL | NULL | NULL | NULL | NULL | 599 | Using where |
| 2 | DEPENDENT SUBQUERY | a | const | PRIMARY | PRIMARY | 2 | const | 1 | Using index |
| 3 | DEPENDENT UNION | b | const | PRIMARY | PRIMARY | 2 | const | 1 | Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+------+--------------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
4 rows in set5、UNION RESULT UNION结果
mysql> explain select * from staff union select * from staff;
+------+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| 1 | PRIMARY | staff | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
| 2 | UNION | staff | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+------+--------------+------------+------+---------------+------+---------+------+------+-----------------+
3 rows in set6、SUBQUERY 子查询中的第一个SELECT语句
mysql> explain select customer_id from customer where store_id =
(select store_id from store where store_id=1);
+----+-------------+----------+-------+-----------------+-----------------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+-----------------+-----------------+---------+-------+------+--------------------------+
| 1 | PRIMARY | customer | ref | idx_fk_store_id | idx_fk_store_id | 1 | const | 326 | Using where; Using index |
| 2 | SUBQUERY | store | const | PRIMARY | PRIMARY | 1 | const | 1 | Using index |
+----+-------------+----------+-------+-----------------+-----------------+---------+-------+------+--------------------------+
2 rows in set有兴趣的可以去把=号换成
in试试
7、DERIVED 派生表的SELECT(FROM子句的子查询)
mysql> explain select * from (select * from customer) a;
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 599 | NULL |
| 2 | DERIVED | customer | ALL | NULL | NULL | NULL | NULL | 599 | NULL |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
2 rows in set8、其它如物化视图等查询自己去造例子去
table(JSON name: table_name)
显示这一行的数据是关于哪张表的，也可以是下列值之一：
unionM,N: The row refers to the union of the rows with id values of M and N.
derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause.
subqueryN: The row refers to the result of a materialized subquery for the row with an id value of N.
partitions (JSON name: partitions)
分区中的记录将被查询相匹配。显示此列仅在使用分区关键字。该值为NULL对于非分区表。
type (JSON name: access_type)
EXPLAIN输出的类型列描述了表的连接方法。下面的列表介绍了连接类型，从最好的类型到最差的命令：
1、system
这是const的一个特例联接类型。表只有一行（=系统表）。
mysql> explain select * from (select * from customer where customer_id=1) a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | NULL |
| 2 | DERIVED | customer | const | PRIMARY | PRIMARY | 2 | const | 1 | NULL |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
2 rows in set2、const
表最多有一个匹配行，它将在查询开始时被读取。因为仅有一行，在这行的列值可被优化器剩余部分认为是常数。const表很快，因为它们只读取一次！
const用于用常数值比较PRIMARY KEY或UNIQUE索引的所有部分时。在下面的查询中，tbl_name可以用于const表：
SELECT * from tbl_name WHERE primary_key=1；
SELECT * from tbl_name WHERE primary_key_part1=1和 primary_key_part2=2；3、eq_ref
对于每个来自于前面的表的行组合，从该表中读取一行。这可能是最好的联接类型，除了const类型。它用在一个索引的所有部分被联接使用并且索引是UNIQUE或PRIMARY KEY。
eq_ref可以用于使用= 操作符比较的带索引的列。比较值可以为常量或一个使用在该表前面所读取的表的列的表达式。
在下面的例子中，MySQL可以使用eq_ref联接来处理ref_tables：
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;# 相对于下面的ref区别就是它使用的唯一索引，即主键或唯一索引，而ref使用的是非唯一索引或者普通索引。id是主键
mysql> explain select a.*,b.* from testa a,testb b where a.id=b.id
;
+----+-------------+-------+--------+---------------+---------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-------------+------+-------------+
| 1 | SIMPLE | b | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| 1 | SIMPLE | a | eq_ref | PRIMARY | PRIMARY | 4 | sakila.b.id | 1 | NULL |
+----+-------------+-------+--------+---------------+---------+---------+-------------+------+-------------+
2 rows in set4、ref
对于每个来自于前面的表的行组合，所有有匹配索引值的行将从这张表中读取。如果联接只使用键的最左边的前缀，或如果键不是UNIQUE或PRIMARY KEY（换句话说，如果联接不能基于关键字选择单个行的话），则使用ref。如果使用的键仅仅匹配少量行，该联接类型是不错的。
ref可以用于使用=或<=>操作符的带索引的列。
在下面的例子中，MySQL可以使用ref联接来处理ref_tables：
SELECT * FROM ref_table WHERE key_column=expr;
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;
SELECT * FROM ref_table,other_table
WHERE ref_table.key_column_part1=other_table.column
AND ref_table.key_column_part2=1;# 使用非唯一性索引或者唯一索引的前缀扫描，返回匹配某个单独值的记录行。name有非唯一性索引
mysql> explain select * from testa where name='aaa';
+----+-------------+-------+------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+----------+---------+-------+------+-----------------------+
| 1 | SIMPLE | testa | ref | idx_name | idx_name | 33 | const | 2 | Using index condition |
+----+-------------+-------+------+---------------+----------+---------+-------+------+-----------------------+
1 row in set
mysql> explain select a.*,b.* from testa a,testb b where a.name=b.cname;
+----+-------------+-------+------+---------------+----------+---------+----------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+----------+---------+----------------+------+-------------+
| 1 | SIMPLE | b | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| 1 | SIMPLE | a | ref | idx_name | idx_name | 33 | sakila.b.cname | 1 | NULL |
+----+-------------+-------+------+---------------+----------+---------+----------------+------+-------------+
2 rows in set5、 fulltext
使用FULLTEXT索引进行联接。
6、ref_or_null
该联接类型如同ref，但是添加了MySQL可以专门搜索包含NULL值的行。在解决子查询中经常使用该联接类型的优化。
在下面的例子中，MySQL可以使用ref_or_null联接来处理ref_tables：
SELECT * FROM ref_table
WHERE key_column=expr OR key_column IS NULL;mysql> explain select * from (select cusno from testa t1,testb t2 where t1.id=t2.id) t where cusno =2 or cusno is null;
+----+-------------+------------+-------------+---------------+-------------+---------+--------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------------+---------------+-------------+---------+--------------+------+--------------------------+
| 1 | PRIMARY | <derived2> | ref_or_null | <auto_key0> | <auto_key0> | 5 | const | 2 | Using where; Using index |
| 2 | DERIVED | t2 | index | PRIMARY | PRIMARY | 4 | NULL | 1 | Using index |
| 2 | DERIVED | t1 | eq_ref | PRIMARY | PRIMARY | 4 | sakila.t2.id | 1 | NULL |
+----+-------------+------------+-------------+---------------+-------------+---------+--------------+------+--------------------------+
3 rows in set此处按照官网的格式未测试出例子来，若有例子的请留言，我测试更新
7、index_merge
该联接类型表示使用了索引合并优化方法。在这种情况下，key列包含了使用的索引的清单，key_len包含了使用的索引的最长的关键元素。
此处按照官网的格式未测试出例子来，若有例子的请留言，我测试更新
8、unique_subquery
unique_subquery是一个索引查找函数，可以完全替换子查询，效率更高。
该类型替换了下面形式的IN子查询的ref：
value IN (SELECT primary_key FROM single_table WHERE some_expr)此处按照官网的格式未测试出例子来，若有例子的请留言，我测试更新
9、index_subquery
该联接类型类似于unique_subquery。可以替换IN子查询，但只适合下列形式的子查询中的非唯一索引：
value IN (SELECT key_column FROM single_table WHERE some_expr)此处按照官网的格式未测试出例子来，若有例子的请留言，我测试更新
10、range
只检索给定范围的行，使用一个索引来选择行。key列显示使用了哪个索引。key_len包含所使用索引的最长关键元素。在该类型中ref列为NULL。
当使用=、<>、>、>=、<、<=、IS NULL、<=>、BETWEEN或者IN操作符，用常量比较关键字列时，可以使用range
SELECT * FROM tbl_name
WHERE key_column = 10;
SELECT * FROM tbl_name
WHERE key_column BETWEEN 10 and 20;
SELECT * FROM tbl_name
WHERE key_column IN (10,20,30);
SELECT * FROM tbl_name
WHERE key_part1 = 10 AND key_part2 IN (10,20,30);11、index
索引类型与ALL类型一样，除了它是走索引树扫描的，它有两种方式：
如果该覆盖索引能满足查询的所有数据，那仅仅扫描这索引树。在这种情况下，Extra列就会显示用Using index。一般仅仅用索引是扫描的比ALL扫描的要快，因为索引树比表数据小很多。
全表扫描被用到从索引中去读取数据， Extra列就不会显示用Using index。
如果查询仅仅是索引列，那MySQL会这个index索引类型
mysql> alter table testa add primary key p_id(id);
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0
mysql> create index idx_name on testa(name);
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0
mysql> insert into testa values(2,2,'aaa');
Query OK, 1 row affected
# 因为查询的列name上建有索引，所以如果这样type走的是index
mysql> explain select name from testa;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | testa | index | NULL | idx_name | 33 | NULL | 2 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set
# 因为查询的列cusno没有建索引，或者查询的列包含没有索引的列，这样查询就会走ALL扫描，如下：
mysql> explain select cusno from testa;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | testa | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set
# *包含有未见索引的列
mysql> explain select * from testa;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | testa | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1