MYSQL explain执行计划
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用explain查看SQL的执行计划
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。
1 查看 select 语句的执行过程
mysql> explain select id,name from test where name=‘clsn‘; +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+ | 1 | SIMPLE | test | ref | name_idx | name_idx | 24 | const | 1 | Using where | +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+ 1 row in set (0.00 sec)
SQL_NO_CACHE的作用是禁止缓存查询结果。
使用where条件查找
mysql> explain select user,host from mysql.user where user=‘root‘ and host=‘127.0.0.1‘; +----+-------------+-------+-------+---------------+---------+---------+-------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------------+------+-------------+ | 1 | SIMPLE | user | const | PRIMARY | PRIMARY | 228 | const,const | 1 | Using index | +----+-------------+-------+-------+---------------+---------+---------+-------------+------+-------------+ 1 row in set (0.00 sec)
2 通过执行计划可以知道什么?
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)
3 MySQL执行计划调用方式
1.EXPLAIN SELECT …… 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
1.5.4 执行计划包含的信息
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
5 id
包含一组数字,表示查询中执行select子句或操作表的顺序
【示例一】id相同,执行顺序由上至下
mysql> 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)
【示例二】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 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)
【示例三】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
mysql> 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)
6 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
说明:
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。
【示例】
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> | 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)
内容说明:
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。
7 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
【示例一】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 | 4 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
【示例二】index:Full Index Scan,index与ALL区别为index类型只遍历索引树
mysql> explain select id from t1; +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | age | 5 | NULL | 4 | Using index | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
【示例三】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。
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 | 3 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.00 sec)
mysql> 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)
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 | 3 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.00 sec)
【示例四】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
mysql> explain select * from t1 where name=‘guo‘; +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+ | 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)
【示例五】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。
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 | 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)
【示例六】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
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 | | | 2 | DERIVED | t1 | const | PRIMARY | PRIMARY | 4 | | 1 | | +----+-------------+------------+--------+---------------+---------+---------+------+------+-------+ 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
【示例七】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。
mysql> 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)
8 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
9 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
【示例】
mysql> 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)
10 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)。
11 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。
12 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。
【示例】
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 | 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)
13 Extra
包含不适合在其他列中显示但十分重要的额外信息
【示例一】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
【示例二】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)
【示例三】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)
【示例四】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)
【示例五】Using join buffer
该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)
删除t1索引
mysql> alter table t1 drop key name; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0
删除t2索引
mysql> alter table t2 drop key name; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0
经常查找
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)
【示例六】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)
【示例七】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)
【示例八】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(...)
14 小结
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。
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