EXPLAIN命令详解
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。需要注意的是,生成的QEP并不确定,它可能会根据很多因素发生改变。MySQL不会将一个QEP和某个给定查询绑定,QEP将由SQL语句每次执行时的实际情况确定,即便使用存储过程也是如此。尽管在存储过程中SQL语句都是预先解析过的,但QEP仍然会在每次调用存储过程的时候才被确定。
通过执行计划可以知道什么?
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
MySQL执行计划调用方式
1.EXPLAIN SELECT ……
变体:
2.EXPLAIN EXTENDED SELECT ……
将执行计划"反编译"成SELECT语句,运行SHOW WARNINGS
可得到被MySQL优化器优化后的查询语句
3.EXPLAIN PARTITIONS SELECT ……
用于分区表的EXPLAIN生成QEP的信息
执行计划包含的信息
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1. id:
包含一组数字,表示查询中执行select子句或操作表的顺序
Example(id相同,执行顺序由上至下)
(root@yayun-mysql-server) [test]>explain select t2.* from t1, t2, t3 where t1.id=t2.id and t1.id=t3.id and t1.name='';
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
| 1 | SIMPLE | t1 | ref | PRIMARY,name | name | 63 | const | 1 | Using where; Using index |
| 1 | SIMPLE | t2 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | |
| 1 | SIMPLE | t3 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
3 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
Example (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)
(root@yayun-mysql-server) [test]>explain select t2.* from t2 where id = (select id from t1 where id = (select t3.id from t3 where t3.name=''));
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
| 3 | SUBQUERY | t3 | ref | name | name | 63 | | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
3 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
Example(id如果相同,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)
(root@yayun-mysql-server) [test]>explain select t2.* from (select t3.id from t3 where t3.name='')s1, t2 where s1.id=t2.id;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | t2 | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 2 | DERIVED | t3 | ref | name | name | 63 | | 1 | Using where; Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
3 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
2. select_type
示查询中每个select子句的类型(简单OR复杂)
a. SIMPLE:查询中不包含子查询或者UNION
b. 查询中若包含任何复杂的子部分,最外层查询则被标记为:PRIMARY
c. 在SELECT或WHERE列表中包含了子查询,该子查询被标记为:SUBQUERY
d. 在FROM列表中包含的子查询被标记为:DERIVED(衍生)用来表示包含在from子句中的子查询的select,mysql会递归执行并将结果放到一个临时表中。服务器内部称为"派生表",因为该临时表是从子查询中派生出来的
e. 若第二个SELECT出现在UNION之后,则被标记为UNION;若UNION包含在FROM子句的子查询中,外层SELECT将被标记为:DERIVED
f. 从UNION表获取结果的SELECT被标记为:UNION RESULT
SUBQUERY和UNION还可以被标记为DEPENDENT和UNCACHEABLE。
DEPENDENT意味着select依赖于外层查询中发现的数据。
UNCACHEABLE意味着select中的某些 特性阻止结果被缓存于一个item_cache中。
Example
(root@yayun-mysql-server) [test]>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> | system | NULL | NULL | NULL | NULL | 0 | const row not found |
| 3 | DERIVED | t1 | ref | name | name | 63 | | 1 | Using where; Using index |
| 2 | SUBQUERY | t3 | index | NULL | age | 5 | NULL | 6 | Using index |
| 4 | UNION | t2 | index | NULL | name | 63 | NULL | 4 | Using index |
| NULL | UNION RESULT | <union1,4> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
5 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
第一行:id列为1,表示第一个select,select_type列的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代表该查询衍生自第三个select查询,即id为3的select。
第二行:id为3,表示该查询的执行次序为2( 4 => 3),是整个查询中第三个select的一部分。因查询包含在from中,所以为derived。
第三行:select列表中的子查询,select_type为subquery,为整个查询中的第二个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操作。
3. type
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, system, NULL
从左到右,性能从最差到最好
Example
a. ALL:Full Table Scan, MySQL将遍历全表以找到匹配的行
(root@yayun-mysql-server) [test]>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 | 4 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)(root@yayun-mysql-server) [test]>
b. index:Full Index Scan,index与ALL区别为index类型只遍历索引树
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
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。显而易见的索引范围扫描是带有between或者where子句里带有<, >查询。当mysql使用索引去查找一系列值时,例如IN()和OR列表,也会显示range(范围扫描),当然性能上面是有差异的。
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>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 | 3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)(root@yayun-mysql-server) [test]>explain select * from t1 where id=1 or id=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 (0.01 sec)(root@yayun-mysql-server) [test]>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 | 3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)(root@yayun-mysql-server) [test]>
d. ref:使用非唯一索引扫描或者唯一索引的前缀扫描,返回匹配某个单独值的记录行
(root@yayun-mysql-server) [test]>explain select * from t1 where name='yayun';
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| 1 | SIMPLE | t1 | ref | name | name | 63 | const | 1 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)(root@yayun-mysql-server) [test]>
e. eq_ref:类似ref,区别就在使用的索引是唯一索引,对于每个索引键值,表中只有一条记录匹配,简单来说,就是多表连接中使用primary key或者 unique key作为关联条件
(root@yayun-mysql-server) [test]>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 | name | 63 | NULL | 4 | Using index |
| 1 | SIMPLE | t2 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
f. const、system:当MySQL对查询某部分进行优化,并转换为一个常量时,使用这些类型访问。如将主键置于where列表中,MySQL就能将该查询转换为一个常量
(root@yayun-mysql-server) [test]>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 | |
| 2 | DERIVED | t1 | const | PRIMARY | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
2 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
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在优化过程中分解语句,执行时甚至不用访问表或索引,例如从一个索引列里选取最小值可以通过单独索引查找完成。
(root@yayun-mysql-server) [test]>explain select * from t1 where id = (select min(id) from t2);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
| 1 | PRIMARY | t1 | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
2 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
4. possible_keys
指出MySQL能使用哪个索引在表中找到记录,查询涉及到的字段上若存在索引,则该索引将被列出,但不一定被查询使用
5. key
显示MySQL在查询中实际使用的索引,若没有使用索引,显示为NULL
Example
(root@yayun-mysql-server) [test]>explain select id,age 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)(root@yayun-mysql-server) [test]>
6. key_len
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)
7. 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
8. rows
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
Example
(root@yayun-mysql-server) [test]>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 | t2 | ref | PRIMARY,name | name | 63 | const | 1 | Using where |
| 1 | SIMPLE | t1 | eq_ref | PRIMARY | PRIMARY | 4 | test.t2.id | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)(root@yayun-mysql-server) [test]>
9. 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)
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
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。
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
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
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
d. Using filesort
MySQL中无法利用索引完成的排序操作称为“文件排序”
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
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。
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>alter table t1 drop key name;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0(root@yayun-mysql-server) [test]>alter table t2 drop key name;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
f. Impossible where
这个值强调了where语句会导致没有符合条件的行。
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
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.
Example
(root@yayun-mysql-server) [test]>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)(root@yayun-mysql-server) [test]>
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。
EXPLAIN命令详解相关推荐
- mysql查询优化explain命令详解
转载自 mysql查询优化explain命令详解 mysql查询优化的方法有很多种,explain是工作当中用的比较多的一种检查方式.explain翻译即解释,就是看mysql语句的查询解释计划,从解 ...
- MySQL调试--explain命令详解
原文网址:MySQL调试--explain命令详解_IT利刃出鞘的博客-CSDN博客 简介 本文介绍MySQL的explain命令的用法及其结果的含义. explain作用 概述 在 SELECT 语 ...
- EXPLAIN 命令详解
转自:https://www.cnblogs.com/gomysql/p/3720123.html 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S ...
- mysql执行计划命令_【MySQL】EXPLAIN命令详解--解释执行计划
具体参考: 原文如下: 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 ...
- MySQL中EXPLAIN命令详解
EXPLAIN显示了MySQL如何使用索引来处理SELECT语句以及连接表.可以帮助选择更好的索引和写出更优化的查询语句. 使用方法,在select语句前加上EXPLAIN就可以了: 如: EXPLA ...
- mysql explain desc_MySQL中EXPLAIN命令详解
explain显示了mysql如何使用索引来处理select语句以及连接表.可以帮助选择更好的索引和写出更优化的查询语句. 使用方法,在select语句前加上explain就可以了: 如: mysql ...
- MySQL Explain命令详解--表的读取顺序,数据读取操作的类型等
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) 不损失精确 ...
- K8s系列之:kubectl子命令详解explain
K8s系列之:kubectl子命令详解explain 资源记录.获取资源及其字段的文档. kubectl explain pods#获取资源的特定字段的文档 kubectl explain pods. ...
- MySQL 数据库 source 命令详解及实例
MySQL 数据库 source 命令详解及实例 MySQL 数据库 source 命令,该命令是数据库导入命令.source 命令的用法非常简单,首先你需要进入 MySQL 数据库的命令行管理界面, ...
最新文章
- iOS GCD_1
- axios队列 vue_(十三 )Vue 封装axios(四种请求)及相关介绍
- 数据结构与算法 | 链表
- linux里的vmware16有中文吗,vmware 16.1专业版_linux版本下载_vmware 16.1下载_5分享
- abovedisplayskip无效_latex公式图片行间距段间距调整心得 -
- bash 后台程序_如何向您的Bash程序添加帮助工具
- 推荐系列:2008年第10期 总12期
- 指针知识(四):指针数学计算
- Windows10系统下PaddleOCR文本检测和文本识别安装
- 开课吧:浅析语音识别算法工程师能力要求
- 开源大数据:Apache Pulsar
- 反恐精英代码_知名网游源代码泄漏 ,外挂潮将来?
- 计算机财务管理模型组成要素,计算机财务管理复习
- IXI数据预处理 + Linux + freesurfer
- 3、TM4之GPIO的输入输出
- Php 类似coffeescript,有什么东西像CoffeeScript for PHP吗?
- C#打印小票自带条形码打印
- 第P9周:YOLOv5-Backbone模块实现
- 联想台式电脑一键恢复后桌面没有计算机了,桌面上的一键恢复快捷图标没有了怎么办?...
- dubbo注册服务和消费服务---入门篇