MySQL EXPLAIN 实践汇总

MySQLEXPLAIN可以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不是最优，这有助于我们对不使用索引的查询进行优化。EXPLAIN对于每个查询语句返回一行信息，它列出了一个有序的表格，MySQL处理语句的时候会读取他们。MySQL解决所有的连接都使用嵌套连接方法。这意味着MySQL读取第一张一行，然后匹配第二张表的所有行，第三张表或更多表都如此。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。

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

语法：

EXPLAIN ; 
  
 
  
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
 
  
 
  
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 |       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------+
 
  
 
  
 
  
#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     |  | 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 |  | ALL  | NULL          | NULL | NULL    | NULL | NULL |       |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
 
  
 
  
#PRIMARY / SUBQUERY
 
  
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 / DEPENDENT SUBQUERY
 
  
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      |  | 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     |  | 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> alter table tabname2 add fulltext(name);
mysql> explain select * from tabname2 where match(name) against('love');
+----+-------------+----------+----------+---------------+------+---------+------+------+-------------+
| id | select_type | table    | type     | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+----------+----------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | tabname2 | fulltext | name          | name | 0       |      |    1 | Using where |
+----+-------------+----------+----------+---------------+------+---------+------+------+-------------+
 
  
 
  
#index_merge(未成功)
 
  
 
  
mysql> explain select * from tabname where tid>1 or indate 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 |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+
 
  
 
  

 
  
#usingjoin buffer
 
  
#Using index for group-by
 
  
 
  

 
  
=====================================================================================
 
  
=====================================================================================
 
  

 
  
现在使用 EXTENDED 情况：
 
  

 
  
语法：
 
  
 
  
EXPLAIN EXTENDED ;

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

show warnings 显示了优化器中是怎么规范表和字段名的，在通过重写和优化规则之后的 select 语句是什么样子。

更多参考：

EXPLAIN Output Format

EXPLAIN EXTENDED Output Format

本文地址：Hello.KK （SQL Server） http://blog.csdn.net/kk185800961/article/details/49179619