mysql执行计划简介

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       |
    +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
    |   | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL |     | 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() 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() null,
indate datetime null,
tid int null,
key(tid),
key(indate)
)engine=myisam;  

insert into tabname(name,indate,tid) values('love',now(),),('lucky',now(),),('passion',now(),);
insert into tabname2(name,indate,tid) values('love',now(),),('lucky',now(),),('passion',now(),); 

#SIMPLE

#PRIMARY / DERIVED

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

#PRIMARY / UNION / UNION RESULT

    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                        |
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+
    |   | PRIMARY     | tabname | const | PRIMARY       | PRIMARY |        | const |     |                              |
    |   | SUBQUERY    | NULL    | NULL  | NULL          | NULL    | NULL    | NULL  | NULL | Select tables optimized away |
    +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+  

#PRIMARY / SUBQUERY

 

#PRIMARY / DEPENDENT SUBQUERY

    mysql> explain select * from tabname a where exists(select  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       |
    +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+
    |   | PRIMARY            | tabname | ALL             | NULL          | NULL    | NULL    | NULL |     | Using where |
    |   | DEPENDENT SUBQUERY | tabname | unique_subquery | PRIMARY       | PRIMARY |        | func |     | 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       |
    +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+
    |   | PRIMARY            | tabname    | ALL    | NULL          | NULL    | NULL    | NULL |     | Using where |
    |   | DEPENDENT SUBQUERY | tabname    | eq_ref | PRIMARY       | PRIMARY |        | func |     | Using index |
    |   | DEPENDENT UNION    | tabname    | eq_ref | PRIMARY       | PRIMARY |        | func |     | Using index |
    | NULL| UNION RESULT      | <union2,> | 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

#const

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

#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 |
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+
    |   | SIMPLE      | a     | ALL    | PRIMARY       | NULL    | NULL    | NULL      |     |       |
    |   | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY |        | test.a.id |     |       |
    +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+  

 

#ref

#ref_or_null

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

 

#fulltext

#index_merge(未成功)

#unique_subquery

#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                    |
    +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+
    |   | PRIMARY            | tabname | ALL            | NULL          | NULL | NULL    | NULL |     | Using where              |
    |   | DEPENDENT SUBQUERY | tabname | index_subquery | tid           | tid  |        | func |     | Using index; Using where |
    +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+  

 

#range

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

#ALL

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

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

#using index

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

 

#using temporary

 

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

 

#not exists

#distinct

#impossible where

#impossible having

mysql> explain select id,count(*) from tabname group by id having =;
mysql> explain select count(*) from tabname having =;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra             |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+
|   | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible HAVING |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+

现在使用 EXTENDED 情况：

语法：

1. EXPLAIN EXTENDED <select statement>;

不使用 extended 和使用extended 的分析情况：

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：

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