MySQL之explain

作者: 郭之源 | 来源:发表于2018-04-29 00:03 被阅读10次

在工作中，我们用于捕捉性能问题最常用的就是打开慢查询，定位执行效率差的SQL，那么当我们定位到一个SQL以后还不算完事，我们还需要知道该SQL的执行计划，比如是全表扫描，还是索引扫描，这些都需要通过EXPLAIN去完成。EXPLAIN命令是查看优化器如何决定执行查询的主要方法。可以帮助我们深入了解MySQL的基于开销的优化器，还可以获得很多可能被优化器考虑到的访问策略的细节，以及当运行SQL语句时哪种策略预计会被优化器采用。

mysql>explain select d1.age, t2.id from (select age,name from t1 where id in (1,2))d1, t2 where d1.age=t2.age group by d1.age, t2.id order by t2.id;
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
| id | select_type | table      | type  | possible_keys | key     | key_len | ref    | rows | Extra                           |
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
|  1 | PRIMARY     | <derived2> | ALL   | NULL          | NULL    | NULL    | NULL   |    2 | Using temporary; Using filesort |
|  1 | PRIMARY     | t2         | ref   | age           | age     | 5       | d1.age |    1 | Using where; Using index        |
|  2 | DERIVED     | t1         | range | PRIMARY       | PRIMARY | 4       | NULL   |    2 | Using where                     |
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
3 rows in set (0.00 sec)

1. id

表示查询中执行select子句或操作表的顺序,越大越先执行,相同从上到下执行

2. select_type

标识查询中每个select子句的简单复杂程度

a.SIMPLE:查询中不包含子查询或UNION查询
b.PRIMARY：标记包含子查询的外层查询
c.SUBQUERY:在select或where中的子查询
d.DERIVED：在from中包含的子查询；子查询中使用union的外层查询
e.UNION：第二个select出现在union之后
f.UNION RESULT：从union表中获取结果的select

SUBQUERY和UNION和可以标记为DEPENDENT和UNCACHEABLE
DEPENDENT意味着select依赖外层查询中发现的数据
UNCACHEABLE意味着select中的默写特性阻止结果被缓存于一个item_cache中

3. table

就是使用的表，这个表可以是用户创建的也可以是临时表或查询结果集（简单）

Example:
mysql> explain select d1.name, ( select id from t3) d2 from (select id,name from t1 where name='')d1 union (select name,id from t2);
+----+--------------+------------+-------+---------------+---------+---------+------+------+-----------------+
| id | select_type  | table      | type  | possible_keys | key     | key_len | ref  | rows | Extra           |
+----+--------------+------------+-------+---------------+---------+---------+------+------+-----------------+
|  1 | PRIMARY      | <derived3> | ALL   | NULL          | NULL    | NULL    | NULL |    2 | NULL            |
|  3 | DERIVED      | t1         | ALL   | NULL          | NULL    | NULL    | NULL |    1 | Using where     |
|  2 | SUBQUERY     | t3         | index | NULL          | PRIMARY | 4       | NULL |    1 | Using index     |
|  4 | UNION        | t2         | ALL   | NULL          | NULL    | NULL    | NULL |    1 | NULL            |
| NULL | UNION RESULT | <union1,4> | ALL   | NULL          | NULL    | NULL    | NULL | NULL | Using temporary |
+----+--------------+------------+-------+---------------+---------+---------+------+------+-----------------+
5 rows in set (0.00 sec)

执行顺序：第四行->第二行->第三行->第一行->第五行
第一行：id列为1，表示第一个select，select_type为primary表示该查询为外层查询，table列被标记为<derived3>，表示查询结果来自一个衍生表，其中3衍生表来自第三个select查询，即id为3的select
第二行：id为3，表示第三个select，包含在from中所以是derived
第三行：select列表的子查询，select_type为subquery,是整张表的第二个select
第四行：select_type为union，说明第四个select是union里的第二个select，最先执行
第五行：代表从union的临时表中读取行的阶段，table列的<union1,4>表示用第一个和第四个select的结果进行union

4. type（重要且不容易）

表示MySQL在表中找到所需行的方式，又称"访问类型"，常见类型如下：
ALL<index<range<ref<eq_ref<const<sysstem<NULL
从左到右，性能从最差到最好
a.ALL:FULL Table Scan，MySQL将遍历全表以找到匹配的行

mysql> explain select * from t1 where email = '';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

b.index:Full Index Scan,遍历索引树

mysql> explain select id from t1;
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | t1_name_index | 257     | NULL |    1 | Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
1 row in set (0.00 sec)

c.range:索引范围扫描，对索引的扫描开始于某一点，返回匹配值域的行。一般where子句使用索引字段betwwen、< 、>、in()和OR列表时候type会是range

mysql> explain select * from t1 where id in (1,4);
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where id between 1 and 4;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    1 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where id <1;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    1 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where id =10 or id= 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 |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where phone like '1831131%';
+----+-------------+-------+-------+-----------------+-----------------+---------+------+------+-----------------------+
| id | select_type | table | type  | possible_keys   | key             | key_len | ref  | rows | Extra                 |
+----+-------------+-------+-------+-----------------+-----------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | t1    | range | t1_phone_unique | t1_phone_unique | 767     | NULL |    1 | Using index condition |
+----+-------------+-------+-------+-----------------+-----------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

d.ref:使用非唯一索引; 唯一索引的前缀扫描，返回匹配某个单独值的记录行（不知道啥意思，看懂的告诉我，哈哈）

mysql> explain select * from t1 where name='yyyy';
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key           | key_len | ref   | rows | Extra                 |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t1    | ref  | t1_name_index | t1_name_index | 257     | const |    1 | Using index condition |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

e.eq_ref:类似ref，多表连接时使用唯一索引或主键连接

mysql> explain select t1.name from t1,t2 where t1.id=t2.id;
+----+-------------+-------+--------+---------------+---------------+---------+------------+------+-------------+
| id | select_type | table | type   | possible_keys | key           | key_len | ref        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------------+---------+------------+------+-------------+
|  1 | SIMPLE      | t1    | index  | PRIMARY       | t1_name_index | 767     | NULL       |    1 | Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY       | 4       | test.t1.id |    1 | Using index |
+----+-------------+-------+--------+---------------+---------------+---------+------------+------+-------------+
2 rows in set (0.00 sec)

f.const、system:当MySQL对查询某部分进行优化，并扎UN哈UN成一个常量时，使用这些类型访问。如将主键置于where列表中，MySQL就能将改查询转化成为一个常量。

mysql> explain select * from (select * from t1 where id=1)b1;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| 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     | t1         | const  | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+

2 rows in set (0.00 sec)
注：system是const类型的特例，当查询的表只有一行的情况下，使用system

g.NULL:MySQL在优化过程中分析语句，有时候甚至不用访问表或索引，例如从一个索引列中选取最小的值可以通过单独的索引查找完成

mysql> explain select * from (select min(id) from t1)b1;
+----+-------------+------------+--------+---------------+------+---------+------+------+------------------------------+
| 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     | NULL       | NULL   | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+------------+--------+---------------+------+---------+------+------+------------------------------+
2 rows in set (0.00 sec)

5.possible_keys（不重要）
显示MySQL可能使用那个索引，但实际不一定使用

6.key （重要）
显示实际使用的索引，如果没有使用索引则为null

mysql> explain select id from t1;
+----+-------------+-------+-------+---------------+-----------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key             | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-----------------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | t1_phone_unique | 767     | NULL |    1 | Using index |
+----+-------------+-------+-------+---------------+-----------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select id,name from t1;
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | t1_name_index | 767     | NULL |    1 | Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
1 row in set (0.00 sec)

key_len（重要）
表示索引中使用的字节数，可通过该列计算查询中使用的索引的长度（key_len显示的值为索引字段的最大可能长度，并非实际使用长度，
即key_len是根据表定义计算而得，不是通过表内检索出的）
ref
表示上述表的连接匹配条件，即哪些列或常量被用于查找索引列上的值
rows（重要）
表示MySQL根据表统计信息及索引选用情况，估算的找到所需的记录所需要读取的行数

mysql> explain select * from t1 , t2 where t1.id=t2.id and t2.name='atlas';
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
|  1 | SIMPLE      | t1    | ALL    | PRIMARY       | NULL    | NULL    | NULL       |    1 | NULL        |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 | Using where |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)

Extra
包含不适合在其他列中显示但十分重要的额外信息
a. Using index
该值表示相应的select操作中使用了覆盖索引（Covering Index）

mysql>explain select id from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

覆盖索引（Covering Index）
MySQL可以利用索引返回select列表中的字段，而不必根据索引再次读取数据文件
包含所有满足查询需要的数据的索引称为覆盖索引（Covering Index）
注意：如果要使用覆盖索引，一定要注意select列表中只取出需要的列，不可select *，因为如果将所有字段一起做索引会导致索引文件过大，查询性能下降

b. Using where
表示mysql服务器将在存储引擎检索行后再进行过滤。许多where条件里涉及索引中的列，当（并且如果）它读取索引时，就能被存储引擎检验，因此不是所有带where字句的查询都会显示"Using where"。有时"Using where"的出现就是一个暗示：查询可受益与不同的索引。

mysql>explain select id,name from t1 where id<4;
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | PRIMARY       | name | 63      | NULL |    4 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

c. Using temporary
表示MySQL需要使用临时表来存储结果集，常见于排序和分组查询

这个值表示使用了内部临时(基于内存的)表。一个查询可能用到多个临时表。有很多原因都会导致MySQL在执行查询期间创建临时表。两个常见的原因是在来自不同表的上使用了DISTINCT,或者使用了不同的ORDER BY和GROUP BY列。可以强制指定一个临时表使用基于磁盘的MyISAM存储引擎。这样做的原因主要有两个：
1)内部临时表占用的空间超过min(tmp_table_size，max_heap_table_size)系统变量的限制
2)使用了TEXT/BLOB 列

mysql>explain select id from t1 where id in (1,2) group by age,name;
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                                        |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where; Using temporary; Using filesort |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
1 row in set (0.00 sec)

d. Using filesort
MySQL中无法利用索引完成的排序操作称为“文件排序”

mysql>explain select id,age from t1 order by name; 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

mysql>explain select id,age from t1 order by age; 
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

e. Using join buffer
改值强调了在获取连接条件时没有使用索引，并且需要连接缓冲区来存储中间结果。如果出现了这个值，那应该注意，根据查询的具体情况可能需要添加索引来改进能。

mysql>explain select t1.name from t1 inner join t2 on t1.name=t2.name;
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref          | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | name          | name | 63      | NULL         |    4 | Using index              |
|  1 | SIMPLE      | t2    | ref   | name          | name | 63      | test.t1.name |    2 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
2 rows in set (0.00 sec)

mysql>alter table t1 drop key name;                                   
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>alter table t2 drop key name; 
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>explain select t1.name from t1 inner join t2 on t1.name=t2.name;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 |                                |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
2 rows in set (0.00 sec)

f. Impossible where
这个值强调了where语句会导致没有符合条件的行。

mysql>EXPLAIN SELECT * FROM t1 WHERE 1=2;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)

h. Select tables optimized away
这个值意味着仅通过使用索引，优化器可能仅从聚合函数结果中返回一行.

mysql>explain select max(id) from t1;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

I. Index merges
当MySQL 决定要在一个给定的表上使用超过一个索引的时候，就会出现以下格式中的一个，详细说明使用的索引以及合并的类型。
Using sort_union(...)
Using union(...)
Using intersect(...)

总结：
• EXPLAIN不会告诉你关于触发器、存储过程的信息或用户自定义函数对查询的影响情况
• EXPLAIN不考虑各种Cache
• EXPLAIN不能显示MySQL在执行查询时所作的优化工作
• 部分统计信息是估算的，并非精确值
• EXPALIN只能解释SELECT操作，其他操作要重写为SELECT后查看执行计划。

参考自Explain 命令详解

MySQL之explain

1. id

2. select_type

3. table

4. type（重要且不容易）

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