mysql 开启日志与性能调优
#查看日期状况
#show variables
like
'%general%'
;
#开启日志
#
SET
GLOBAL
general_log =
'On'
;
#指定日志文件
#
SET
GLOBAL
general_log_file =
'/var/lib/mysql/mysql.log'
;
原文以下：mysql
在工做中，咱们用于捕捉性能问题最经常使用的就是打开慢查询，定位执行效率差的SQL，那么当咱们定位到一个SQL之后还不算完事，咱们还须要知道该SQL的执行计划，好比是全表扫描，仍是索引扫描，这些都须要经过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。能够帮助咱们深刻了解MySQL的基于开销的优化器，还能够得到不少可能被优化器考虑到的访问策略的细节，以及当运行SQL语句时哪一种策略预计会被优化器采用。须要注意的是，生成的QEP并不肯定，它可能会根据不少因素发生改变。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状况肯定，即使使用存储过程也是如此。尽管在存储过程当中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肯定。sql
经过执行计划能够知道什么？缓存
(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
可获得被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的信息服务器
执行计划包含的信息函数
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1. id：性能
包含一组数字，表示查询中执行select子句或操做表的顺序优化
Example（id相同，执行顺序由上至下）spa
(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 （若是是子查询，id的序号会递增，id值越大优先级越高，越先被执行）日志
(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若是相同，能够认为是一组，从上往下顺序执行；在全部组中，id值越大，优先级越高，越先执行）code
(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
示查询中每一个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"，由于该临时表是从子查询中派生出来的
e. 若第二个SELECT出如今UNION以后，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还能够被标记为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表明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，因此为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，最早执行。
第五行：表明从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操做。
3. type
表示MySQL在表中找到所需行的方式，又称“访问类型”，常见类型以下:
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
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 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）,固然性能上面是有差别的。
(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：使用非惟一索引扫描或者惟一索引的前缀扫描，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：相似ref，区别就在使用的索引是惟一索引，对于每一个索引键值，表中只有一条记录匹配，简单来讲，就是多表链接中使用primary key或者 unique key做为关联条件
(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状况下，使用system
g. NULL：MySQL在优化过程当中分解语句，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能够经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能使用哪一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，但不必定被查询使用
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，可经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，并不是实际使用长度，即key_len是根据表定义计算而得，不是经过表内检索出的）
7. ref
表示上述表的链接匹配条件，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状况，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
包含不适合在其余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操做中使用了覆盖索引（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能够利用索引返回select列表中的字段，而没必要根据索引再次读取数据文件
包含全部知足查询须要的数据的索引称为覆盖索引（Covering Index）
注意：若是要使用覆盖索引，必定要注意select列表中只取出须要的列，不可select *，由于若是将全部字段一块儿作索引会致使索引文件过大，查询性能降低
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（而且若是）它读取索引时，就能被存储引擎检验，所以不是全部带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：查询可受益与不一样的索引。
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须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。有不少缘由都会致使MySQL在执行查询期间建立临时表。两个常见的缘由是在来自不一样表的上使用了DISTINCT,或者使用了不一样的ORDER BY和GROUP BY列。能够强制指定一个临时表使用基于磁盘的MyISAM存储引擎。这样作的缘由主要有两个：
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中没法利用索引完成的排序操做称为“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
改值强调了在获取链接条件时没有使用索引，而且须要链接缓冲区来存储中间结果。若是出现了这个值，那应该注意，根据查询的具体状况可能须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语句会致使没有符合条件的行。
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
这个值意味着仅经过使用索引，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，就会出现如下格式中的一个，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状况• EXPLAIN不考虑各类Cache• EXPLAIN不能显示MySQL在执行查询时所做的优化工做• 部分统计信息是估算的，并不是精确值• EXPALIN只能解释SELECT操做，其余操做要重写为SELECT后查看执行计划。
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