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MySQL EXPLAIN 实践汇总

2015-10-16 16:42 603 查看
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 <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

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     | <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 |       |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+


#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      | <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_column
FROMsingle_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> 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<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 H***ING |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+


#usingjoin buffer
#Using index for group-by

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

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

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


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

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