MySQL执行计划
在企业的应用场景中，为了知道优化SQL语句的执行，需要查看SQL语句的具体执行过程，以加快SQL语句的执行效率。
可以使用explain+SQL语句来模拟优化器执行SQL查询语句，从而知道mysql是如何处理sql语句的。
官网地址： https://dev.mysql.com/doc/refman/5.7/en/explain-output.html
执行计划中包含的信息如下：
Column | Meaning |
id | The |
select_type | The |
table | The table for the output row |
partitions | The matching partitions |
type | The join type |
possible_keys | The possible indexes to choose |
key | The index actually chosen |
key_len | The length of the chosen key |
ref | The columns compared to the index |
rows | Estimate of rows to be examined |
filtered | Percentage of rows filtered by table condition |
extra | Additional information |
id
select查询的序列号，包含一组数字，表示查询中执行select子句或者操作表的顺序。
id号分为三种情况：
1、如果id相同，那么执行顺序从上到下。
mysql> explain select * from emp e join dept d on e.deptno = d.deptno;
+----+-------------+-------+------------+------+---------------+-----------+---------+-----------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----------+---------+-----------------+------+----------+-------+
| 1 | SIMPLE | d | NULL | ALL | PRIMARY | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 1 | SIMPLE | e | NULL | ref | fk_deptno | fk_deptno | 5 | oracle.d.deptno | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-----------+---------+-----------------+------+----------+-------+2、如果id不同，如果是子查询，id的序号会递增，id值越大优先级越高，越先被执行。
mysql> explain select * from emp e where exists (select d.deptno from dept d where d.dname = 'SALES' and d.deptno=e.deptno);
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------------+
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | d | NULL | eq_ref | PRIMARY | PRIMARY | 4 | oracle.e.deptno | 1 | 25.00 | Using where |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------------+3、id相同和不同的同时存在：相同的可以认为是一组，从上往下顺序执行，在所有组中，id值越大，优先级越高，越先执行。
mysql> explain select * from emp e join salgrade sg on e.sal between sg.losal and sg.hisal where exists (select d.deptno from dept d where d.dname = 'SALES' and d.deptno=e.deptno);
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+----------------------------------------------------+
| 1 | PRIMARY | sg | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | NULL |
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 11.11 | Using where; Using join buffer (Block Nested Loop) |
| 2 | DEPENDENT SUBQUERY | d | NULL | eq_ref | PRIMARY | PRIMARY | 4 | oracle.e.deptno | 1 | 25.00 | Using where |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+----------------------------------------------------+select_type
select_type主要用来分辨查询的类型，是普通查询还是联合查询还是子查询。
select_type Value | Meaning |
SIMPLE | Simple SELECT (not using UNION or subqueries) |
PRIMARY | Outermost SELECT |
UNION | Second or later SELECT statement in a UNION |
DEPENDENT UNION | Second or later SELECT statement in a UNION, dependent on outer query |
UNION RESULT | Result of a UNION. |
SUBQUERY | First SELECT in subquery |
DEPENDENT SUBQUERY | First SELECT in subquery, dependent on outer query |
DERIVED | Derived table |
UNCACHEABLE SUBQUERY | A subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query |
UNCACHEABLE UNION | The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY) |
simple：简单查询，不包含子查询和union。
mysql> explain select * from emp;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | emp | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+primary：查询中若包含任何复杂的子查询，最外层查询则被标记为Primary（不是主键查询的意思）。
dependent subquery：子查询要受到外部表查询的影响。
mysql> explain select * from emp e where exists (select d.deptno from dept d where d.dname = 'SALES' and d.deptno=e.deptno);
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------------+
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | d | NULL | eq_ref | PRIMARY | PRIMARY | 4 | oracle.e.deptno | 1 | 25.00 | Using where |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------------+union：若第二个select出现在union之后，则被标记为union。
union result：从union表获取结果的select。
mysql> explain select * from emp where deptno = 10 union select * from emp where sal >2000;
+----+--------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-----------------+
| 1 | PRIMARY | emp | NULL | ref | fk_deptno | fk_deptno | 5 | const | 3 | 100.00 | NULL |
| 2 | UNION | emp | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 33.33 | Using where |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-----------------+dependent union：跟union类似，此处的depentent表示union或union all联合而成的结果会受外部表影响。
mysql> explain select * from emp e where e.empno in ( select empno from emp where deptno = 10 union select empno from emp where sal >2000);
+----+--------------------+------------+------------+--------+-------------------+---------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+--------+-------------------+---------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | emp | NULL | eq_ref | PRIMARY,fk_deptno | PRIMARY | 4 | func | 1 | 21.43 | Using where |
| 3 | DEPENDENT UNION | emp | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 33.33 | Using where |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------------+--------+-------------------+---------+---------+------+------+----------+-----------------+subquery:在select或者where列表中包含子查询。
mysql> explain select * from emp where sal > (select avg(sal) from emp) ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 33.33 | Using where |
| 2 | SUBQUERY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+DERIVED: from子句中出现的子查询，也叫做派生类。
mysql> explain select * from (select deptno,max(sal) from emp group by deptno) t;
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | NULL |
| 2 | DERIVED | emp | NULL | index | fk_deptno | fk_deptno | 5 | NULL | 14 | 100.00 | NULL |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-------+UNCACHEABLE SUBQUERY：表示使用子查询的结果不能被缓存。
mysql> explain select * from emp where empno = (select empno from emp where deptno=@@sort_buffer_size);
+----+----------------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+----------------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
| 2 | UNCACHEABLE SUBQUERY | emp | NULL | ref | fk_deptno | fk_deptno | 5 | const | 1 | 100.00 | Using index |
+----+----------------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------------+uncacheable union：表示union的查询结果不能被缓存，sql语句未验证。
table
table对应行正在访问哪一个表，表名或者别名，可能是临时表或者union合并结果集。
- 如果是具体的表名，则表明从实际的物理表中获取数据，当然也可以是表的别名。
- 表名是derivedN的形式，表示使用了id为N的查询产生的衍生表。
- 当有union result的时候，表名是union n1,n2等的形式，n1,n2表示参与union的id。
type
type显示的是访问类型，访问类型表示我是以何种方式去访问我们的数据，最容易想的是全表扫描，直接暴力的遍历一张表去寻找需要的数据，效率非常低下，访问的类型有很多，效率从最好到最坏依次是：
system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
一般情况下，得保证查询至少达到range级别，最好能达到ref。
all：全表扫描，一般情况下出现这样的sql语句而且数据量比较大的话那么就需要进行优化。
mysql> explain select * from city;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | city | NULL | ALL | NULL | NULL | NULL | NULL | 600 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+index：全索引扫描这个比all的效率要好，主要有两种情况，一种是当前的查询时覆盖索引，即我们需要的数据在索引中就可以索取，或者是使用了索引进行排序，这样就避免数据的重排序。
mysql> explain select empno from emp;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | index | NULL | fk_deptno | 5 | NULL | 14 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+range：表示利用索引查询的时候限制了范围，在指定范围内进行查询，这样避免了index的全索引扫描，适用的操作符： =, <>, >, >=, <, <=, IS NULL, BETWEEN, LIKE, or IN()。
mysql> explain select * from emp where empno between 7000 and 7500;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 2 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+index_subquery：利用索引来关联子查询，不再扫描全表。
unique_subquery：该连接类型类似与index_subquery，使用的是唯一索引。
index_merge：在查询过程中需要多个索引组合使用，没有模拟出来。
ref_or_null：对于某个字段即需要关联条件，也需要null值的情况下，查询优化器会选择这种访问方式。
ref：使用了非唯一性索引进行数据的查找。
eq_ref ：使用唯一性索引进行数据查找。
const：这个表至多有一个匹配行。
mysql> explain select * from emp where empno = 7369;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | emp | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+system：表只有一行记录（等于系统表），这是const类型的特例，平时不会出现。
possible_keys
显示可能应用在这张表中的索引，一个或多个，查询涉及到的字段上若存在索引，则该索引将被列出，但不一定被查询实际使用。
key
实际使用的索引，如果为null，则没有使用索引，查询中若使用了覆盖索引，则该索引和查询的select字段重叠。
key_len
key_len表示索引中使用的字节数，可以通过key_len计算查询中使用的索引长度，在不损失精度的情况下长度越短越好。
ref
ref显示索引的哪一列被使用了，如果可能的话，是一个常数。
rows
根据表的统计信息及索引使用情况，大致估算出找出所需记录需要读取的行数，此参数很重要，直接反应的sql找了多少数据，在完成目的的情况下越少越好。
extra
包含额外的信息。
using filesort：说明mysql无法利用索引进行排序，只能利用排序算法进行排序，会消耗额外的位置。
mysql> explain select * from emp order by sal;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | emp | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+using temporary：建立临时表来保存中间结果，查询完成之后把临时表删除。
mysql> explain select ename,count(*) from emp where deptno = 10 group by ename;
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------------------------------------+
| 1 | SIMPLE | emp | NULL | ref | fk_deptno | fk_deptno | 5 | const | 3 | 100.00 | Using index condition; Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------------------------------------+using index：这个表示当前的查询是覆盖索引的，直接从索引中读取数据，而不用访问数据表（回表）。如果同时出现using where表明索引被用来执行索引键值的查找，如果没有，表明索引被用来读取数据，而不是真的查找。
mysql> explain select deptno,count(*) from emp group by deptno limit 10;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | index | fk_deptno | fk_deptno | 5 | NULL | 10 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+using where:使用where进行条件过滤。
mysql> explain select * from emp where empno > 7000;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 14 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+using join buffer：使用连接缓存。
mysql> explain select * from emp e join salgrade sg on e.sal between sg.losal and sg.hisal where exists (select d.deptno from dept d where d.dname = 'SALES' and d.deptno=e.deptno);
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+----------------------------------------------------+
| 1 | PRIMARY | sg | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | NULL |
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 14 | 11.11 | Using where; Using join buffer (Block Nested Loop) |
| 2 | DEPENDENT SUBQUERY | d | NULL | eq_ref | PRIMARY | PRIMARY | 4 | oracle.e.deptno | 1 | 25.00 | Using where |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+----------------------------------------------------+no matching row in const table：where语句的结果总是false。
mysql> explain select * from emp where empno = 7469;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+公众号：架构师升级之路
