<span>mysql执行计划看是否最优</span>

介绍

　　本篇主要通过汇总网上的大牛的知识，简单介绍一下如何使用mysql的执行计划，并根据执行计划判断如何优化和是否索引最优。

　　执行计划可显示估计查询语句执行计划，从中可以分析查询的执行情况是否最优，有助于对不使用索引的语句进行优化。EXPLAIN对每个查询返回一行信息，列出了有序的表格，MySQL处理语句的时候读取他们。MySQL解决所有的连接使用嵌套连接方法。这意味读取第一张一行，然后匹配第二张表的所有行，第三张表甚至更多表。当所有的表在处理时，MySQL会输出已经查询出来的列，并且回溯到表继续查找直到所有的行被找到，从该表读取下一行，直到程序继续处理下一张表。

使用关键词 EXTENDED ，EXPLAIN 会处理通过 SHOW WARNINGS 看到的一些额外信息。EXPLAIN EXTENDED 会显示这些滤出的列。

语法：

EXPLAIN <select statement>;

输出表格字段如下：

    mysql> explain select * from mysql.user where user='root';  
    +----+-------------+-------+------+---------------+------+---------+------+------+-------------+  
    | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |  
    +----+-------------+-------+------+---------------+------+---------+------+------+-------------+  
    |  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |  
    +----+-------------+-------+------+---------------+------+---------+------+------+-------------+

Column	JSONName	Meaning
`id`	`select_id`	查询标识。id越大优先执行；id相同自上而下执行；
`select_type`	None	查询的类型
`table`	`table_name`	查询的表
`partitions`	`partitions`	Thematching partitions
`type`	`access_type`	连接类型
`possible_keys`	`possible_keys`	可能选择的索引
`key`	`key`	实际使用的索引
`key_len`	`key_length`	使用的索引长度
`ref`	`ref`	哪一列或常数在查询中与索引键列一起使用
`rows`	`rows`	估计查询的行数
`filtered`	`filtered`	被条件过滤掉的行数百分比
`Extra`	None	解决查询的一些额外信息

以下主要举例说明3个字段：select_type 、type、Extra

【select_type】

alue	JSONName	Meaning
`SIMPLE`	None	`简单查询` (不使用`UNION`或子查询)
`PRIMARY`	None	`外层查询，主查询`
`UNION`	None	`UNION中`第二个语句或后面的语句
`DEPENDENTUNION`	`dependent` (`true`)	`UNION中第二个语句或后面的语句`,独立于外部查询
`UNIONRESULT`	`union_result`	`UNION的结果`
`SUBQUERY`	None	子查询中第一个SELECT
`DEPENDENTSUBQUERY`	`dependent` (`true`)	子查询中第一个SELECT,独立于外部查询
`DERIVED`	None	子查询在 FROM子句中
`MATERIALIZED`	`materialized_from_subquery`	物化子查询（不清楚是什么样的查询语句？）
`UNCACHEABLESUBQUERY`	`cacheable` (`false`)	结果集不能被缓存的子查询，必须重新评估外层查询的每一行
`UNCACHEABLEUNION`	`cacheable` (`false`)	`UNION中第二个语句或后面的语句属于不可缓存的子查询`

创建测试表：

create table tabname (  
id int auto_increment not null primary key,  
name varchar(10) null,  
indate datetime null,  
tid int null,  
key(tid),  
key(indate)  
)engine=innodb;  
  
  
create table tabname2 (  
id int auto_increment not null primary key,  
name varchar(10) null,  
indate datetime null,  
tid int null,  
key(tid),  
key(indate)  
)engine=myisam;  
  
  
insert into tabname(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);  
insert into tabname2(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);

#SIMPLE

create table tabname (  
id int auto_increment not null primary key,  
name varchar(10) null,  
indate datetime null,  
tid int null,  
key(tid),  
key(indate)  
)engine=innodb;  
  
  
create table tabname2 (  
id int auto_increment not null primary key,  
name varchar(10) null,  
indate datetime null,  
tid int null,  
key(tid),  
key(indate)  
)engine=myisam;  
  
  
insert into tabname(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);  
insert into tabname2(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);

#PRIMARY / DERIVED

    mysql> explain select * from (select * from tabname) as a;  
    +----+-------------+------------+------+---------------+------+---------+------+------+-------+  
    | id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |  
    +----+-------------+------------+------+---------------+------+---------+------+------+-------+  
    |  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |  
    |  2 | DERIVED     | tabname    | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |  
    +----+-------------+------------+------+---------------+------+---------+------+------+-------+

#PRIMARY / UNION / UNION RESULT

    mysql> explain select * from tabname union select * from tabname;  
    mysql> explain select * from tabname union all select * from tabname;  
    +----+--------------+------------+------+---------------+------+---------+------+------+-------+  
    | id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |  
    +----+--------------+------------+------+---------------+------+---------+------+------+-------+  
    |  1 | PRIMARY      | tabname    | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |  
    |  2 | UNION        | tabname    | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |  
    | NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL |       |  
    +----+--------------+------------+------+---------------+------+---------+------+------+-------+

    mysql> explain select * from tabname where id=(select max(id) from tabname);  
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+  
    | id | select_type | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra                        |  
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+  
    |  1 | PRIMARY     | tabname | const | PRIMARY       | PRIMARY | 4       | const |    1 |                              |  
    |  2 | SUBQUERY    | NULL    | NULL  | NULL          | NULL    | NULL    | NULL  | NULL | Select tables optimized away |  
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+

#PRIMARY / SUBQUERY

#PRIMARY / DEPENDENT SUBQUERY

[plain]
view plain
copy

    mysql> explain select * from tabname a where exists(select 1 from tabname b where a.id=b.id);  
    mysql> explain select *,(select name from tabname b where a.id=b.id) from tabname a;  
    mysql> explain select * from tabname where id not in(select id from tabname);  
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+  
    | id | select_type        | table   | type            | possible_keys | key     | key_len | ref  | rows | Extra       |  
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+  
    |  1 | PRIMARY            | tabname | ALL             | NULL          | NULL    | NULL    | NULL |    3 | Using where |  
    |  2 | DEPENDENT SUBQUERY | tabname | unique_subquery | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |  
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+

#PRIMARY / DEPENDENT UNION / DEPENDENT SUBQUERY / UNION RESULT

    mysql> explain select * from tabname where id in (select id from tabname union select id from tabname);  
    +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+  
    | id | select_type        | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra       |  
    +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+  
    |  1 | PRIMARY            | tabname    | ALL    | NULL          | NULL    | NULL    | NULL |    3 | Using where |  
    |  2 | DEPENDENT SUBQUERY | tabname    | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |  
    |  3 | DEPENDENT UNION    | tabname    | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |  
    | NULL| UNION RESULT      | <union2,3> | ALL    | NULL          | NULL    | NULL    | NULL | NULL |             |  
    +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+

【type】

type	Meaning
`system`	表仅一行数据 (=system table).这是const连接类型的特例。
`const`	`表最多只有一个匹配行，在查询开始时被读取。因为只有一个值，优化器将该列值视为常量。当在primarykey或者unique索引作为常量比较时被使用。`
`eq_ref(engine=myisam)`	`来自前面表的结果集中读取一行，这是除system和const外最好的连接类型。当在使用PRIMARYKEY或者UNIQUENOT NULL的索引时会被使用。`
`ref`	对于前面表的结果集匹配查询的所有行，当连接使用索引key时，或者索引不是`PRIMARYKEY和UNIQUE，`则使用该类型。如果使用索引匹配少量行时，是不错的连接类型。
`ref_or_null`	`连接类型类似ref，只是搜索的行中包含NULL值MySQL做了额外的查找。`
`fulltext`	使用全文索引时出现。
`index_merge`	使用了索引合并优化。(未成功)
`unique_subquery`	该类型将ref替换成以下子查询的格式： valueIN (SELECTprimary_key FROMsingle_table WHERE some_expr)
`index_subquery`	与 `unique_subquery类似，但是将主键改为非唯一索引：` valueIN (SELECTkey_columnFROMsingle_table WHERE some_expr)
`range`	使用索引检索给定范围内的行。
`index`	`该连接类型与ALL相同，除了扫描索引树。如果查询的字段都在索引列中，则使用index类型，否则为ALL类型。`
`ALL`	对于前面表的结果集中，进行了全表扫描。最差的一种类型，应考虑查询优化了！

查询类型性能由优到差：

system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

#system

    mysql> explain select id from(select id from tabname where id=1) as a;  
    +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+  
    | id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra       |  
    +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+  
    |  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL |    1 |             |  
    |  2 | DERIVED     | tabname    | const  | PRIMARY       | PRIMARY | 4       |      |    1 | Using index |  
    +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+

#const

    mysql> explain select * from tabname as a,tabname as b where a.id=b.id and a.id=1;  
    mysql> explain select * from tabname where id=1;  
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+  
    | id | select_type | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra |  
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+  
    |  1 | SIMPLE      | tabname | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |  
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

#eq_ref(engine=myisam)

    mysql> explain select * from tabname2 as a,tabname2 as b where a.id=b.id;  
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+  
    | id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | Extra |  
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+  
    |  1 | SIMPLE      | a     | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    3 |       |  
    |  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | test.a.id |    1 |       |  
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+

#ref

    mysql> explain select * from tabname as a,tabname as b where a.tid=b.tid and a.tid=2;  
    mysql> explain select * from tabname where tid=2;  
    +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+  
    | id | select_type | table   | type | possible_keys | key  | key_len | ref   | rows | Extra       |  
    +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+  
    |  1 | SIMPLE      | tabname | ref  | tid           | tid  | 5       | const |    1 | Using where |  
    +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+

#ref_or_null

    mysql> explain select id,tid from tabname where tid=2 or tid is null;  
    +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+  
    | id | select_type | table   | type        | possible_keys | key  | key_len | ref   | rows | Extra                    |  
    +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+  
    |  1 | SIMPLE      | tabname | ref_or_null | tid           | tid  | 5       | const |    2 | Using where; Using index |  
    +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+

#fulltext

    mysql> explain select id,tid from tabname where tid=2 or tid is null;  
    +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+  
    | id | select_type | table   | type        | possible_keys | key  | key_len | ref   | rows | Extra                    |  
    +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+  
    |  1 | SIMPLE      | tabname | ref_or_null | tid           | tid  | 5       | const |    2 | Using where; Using index |  
    +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+

#index_merge(未成功)

mysql> explain select * from tabname where tid>1 or indate<now();  
mysql> explain select * from tabname where (tid>1 or indate>now()) AND name<'kk';

#unique_subquery

    mysql> explain select * from tabname where tid in(select id from tabname);  
    mysql> explain select * from tabname where id in(select id from tabname);  
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+  
    | id | select_type        | table   | type            | possible_keys | key     | key_len | ref  | rows | Extra       |  
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+  
    |  1 | PRIMARY            | tabname | ALL             | NULL          | NULL    | NULL    | NULL |    3 | Using where |  
    |  2 | DEPENDENT SUBQUERY | tabname | unique_subquery | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |  
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+

#index_subquery

    mysql> explain select * from tabname where tid in(select tid from tabname);  
    mysql> explain select * from tabname where id in(select tid from tabname);  
    +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+  
    | id | select_type        | table   | type           | possible_keys | key  | key_len | ref  | rows | Extra                    |  
    +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+  
    |  1 | PRIMARY            | tabname | ALL            | NULL          | NULL | NULL    | NULL |    3 | Using where              |  
    |  2 | DEPENDENT SUBQUERY | tabname | index_subquery | tid           | tid  | 5       | func |    1 | Using index; Using where |  
    +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+

#range

    mysql> explain select * from tabname where tid between 1 and 2;  
    mysql> explain select * from tabname where id>1;  
    +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+  
    | id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra       |  
    +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+  
    |  1 | SIMPLE      | tabname | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |  
    +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

#index

mysql> explain select id,tid from tabname;  
mysql> explain select tid from tabname;  
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |  
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
|  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 | Using index |  
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+

#ALL

    mysql> explain select * from tabname where tid<>2;  
    mysql> explain select * from tabname;  
    +----+-------------+---------+------+---------------+------+---------+------+------+-------+  
    | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra |  
    +----+-------------+---------+------+---------------+------+---------+------+------+-------+  
    |  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |  
    +----+-------------+---------+------+---------------+------+---------+------+------+-------+

【Extra】

该列输出关MySQL如何解决查询的额外信息。（下面列出部分常见的）

Extra	Meaning
`usingwhere`	使用过滤条件
`usingindex`	`从索引树中查找所有列`
`usingtemporary`	`使用临时表存储结果集，在使用groupby和orderby发生`
`selecttables optimized away`	没有groupby情况下使用min(),max(),或者count(*)
`usingfilesort`	`有排序`
`notexists`	在leftjoin中匹配一行之后将不再继续查询查询
`distinct`	查找到第一个匹配的行之后，MySQL则会停止对当前行的搜索
`impossiblewhere`	where子句总数失败的查询
`impossiblehaving`	`having子句总数失败的查询`
`usingjoin buffer`	使用连接缓存
`Usingindex for group-by`	`与Usingindex类似，在使用group-by时可从索引中找到字段`

#using where

    mysql> explain select * from tabname where id>2;  
    mysql> explain select * from tabname where tid=2;  
    +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+  
    | id | select_type | table   | type | possible_keys | key  | key_len | ref   | rows | Extra       |  
    +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+  
    |  1 | SIMPLE      | tabname | ref  | tid           | tid  | 5       | const |    1 | Using where |  
    +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+

#using index

    mysql> explain select tid from tabname;  
    +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
    | id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |  
    +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
    |  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 | Using index |  
    +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+

#using temporary

    mysql> explain select distinct name from tabname;  
    +----+-------------+---------+------+---------------+------+---------+------+------+-----------------+  
    | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra           |  
    +----+-------------+---------+------+---------------+------+---------+------+------+-----------------+  
    |  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using temporary |  
    +----+-------------+---------+------+---------------+------+---------+------+------+-----------------+

#select tables optimized away

    mysql> explain select max(tid) from tabname;  
    +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+  
    | 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 |  
    +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

#using filesort

    mysql> explain select id,name from tabname group by id,name;  
    mysql> explain select * from tabname order by name;  
    +----+-------------+---------+------+---------------+------+---------+------+------+----------------+  
    | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra          |  
    +----+-------------+---------+------+---------------+------+---------+------+------+----------------+  
    |  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using filesort |  
    +----+-------------+---------+------+---------------+------+---------+------+------+----------------+

#not exists

    mysql> explain select * from tabname a left join tabname b on a.id=b.id where b.id is null;  
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------------------------+  
    | id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | Extra                   |  
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------------------------+  
    |  1 | SIMPLE      | a     | ALL    | NULL          | NULL    | NULL    | NULL      |    3 |                         |  
    |  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | test.a.id |    1 | Using where; Not exists |  
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------------------------+

#distinct

mysql> explain select distinct a.id from tabname a left join tabname b on a.id=b.id;  
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------------------------------+  
| id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | Extra                        |  
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------------------------------+  
|  1 | SIMPLE      | a     | index  | NULL          | tid     | 5       | NULL      |    3 | Using index; Using temporary |  
|  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | test.a.id |    1 | Using index; Distinct        |  
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------------------------------+

#impossible where

    mysql> explain select * from tabname 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 |  
    +----+-------------+-------+------+---------------+------+---------+------+------+------------------+

#impossible having

mysql> explain select id,count(*) from tabname group by id having 1=2;  
mysql> explain select count(*) from tabname having 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 HAVING |  
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+

现在使用 EXTENDED 情况：

语法：

EXPLAIN EXTENDED <select statement>;

不使用 extended 和使用extended 的分析情况：

    mysql> explain select tid from tabname;  
    +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
    | id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |  
    +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
    |  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 | Using index |  
    +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+  
    1 row in set (0.00 sec)  
      
    mysql> explain extended select tid from tabname;  
    +----+-------------+---------+-------+---------------+------+---------+------+------+----------+-------------+  
    | id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |  
    +----+-------------+---------+-------+---------------+------+---------+------+------+----------+-------------+  
    |  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 |   100.00 | Using index |  
    +----+-------------+---------+-------+---------------+------+---------+------+------+----------+-------------+  
    1 row in set, 1 warning (0.00 sec)

可以看到，使用 extended 时，输出的最下面多了 1 条警告。此时可以用 show warnings 来查看：

    mysql> show warnings \G;  
    *************************** 1. row ***************************  
      Level: Note  
       Code: 1003  
    Message: select `test`.`tabname`.`tid` AS `tid` from `test`.`tabname`  
    1 row in set (0.00 sec)  
      
    ERROR:  
    No query specified