Explain语句

序言

​ Explain语句提供了mysql如何执行语句的信息，包括select,delete,insert, replace, update

Explain输出的列

Column	JSON Name	Meaning
id	select_id	The SELECT identifier
select_type	None	The SELECT type
table	table_name	The table for the output row
partitions	partitions	The matching partitions
type	access_type	The join type
possible_keys	possible_keys	The possible indexes to choose
key	key	The index actually chosen
key_len	key_length	The length of the chosen key
ref	ref	The columns compared to the index
rows	rows	Estimate(估算) of rows to be examined(检查)
filtered	filtered	Percentage(百分比) of rows filtered(过滤) by table condition
Extra	None	Additional information

select_type

select的类型，包含如下值:

Value	Meaning
SIMPLE	Simple SELECT (not using UNION or subqueries)
PRIMARY	Outermost SELECT
UNION	Second or later SELECT statement in a UNION
DEPENDENT UNION	Second or later SELECT statement in a UNION, dependent on outer query
UNION RESULT	Result of a UNION.
SUBQUERY	First SELECT in subquery
DEPENDENT SUBQUERY	First SELECT in subquery, dependent on outer query
DERIVED	Derived table
DEPENDENT DERIVED	Derived table dependent on another table
MATERIALIZED	Materialized subquery
UNCACHEABLE SUBQUERY	A subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query
UNCACHEABLE UNION	The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY)

type

​ 顺序从好到差依次介绍如下:

• system：表只有一行。system是const的特例

• const：const用于将主键或唯一索引的所有部分与常量值进行=测试，表最多有一个匹配行

  SELECT * FROM tbl_name WHERE primary_key=1;
  
  SELECT * FROM tbl_name
    WHERE primary_key_part1=1 AND primary_key_part2=2;
    
  --上述查询是const类型的 
  

• eq_ref：多表联结中使用primary key或者 unique not null的列，进行=测试，比较值可以是常量，也可以是在该表之前读取的表中的列的表达式

  SELECT * FROM ref_table,other_table
    WHERE ref_table.key_column=other_table.column;
  
  SELECT * FROM ref_table,other_table
    WHERE ref_table.key_column_part1=other_table.column
    AND ref_table.key_column_part2=1;
    
  --上述查询是eq_ref类型的 
  

  create table table_0(id_0 bigint primary key, v_0 int, index(v_0));
  create table table_1(id_1 bigint primary key, v_1 int, index(v_1));
  
  insert into table_0(id_0, v_0) values (0, 1), (1, 2);
  insert into table_1(id_1, v_1) values (0, 1), (1, 2);
  
  explain SELECT * from table_0, table_1 where table_0.id_0 = table_1.id_1
  

  id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	extra
  1	simple	table_0	null	index	primary	v_0	5	null	2	100	using index
  1	simple	table_1	null	eq_ref	primary	primary	8	szn.table_0.id+0	1	100	null

• ref：类似于eq_ref，ref操作的索引不是primary key或者 unique not null，所以返回的行数可能大于1行(在索引列上使用的依然是=操作)

  SELECT * FROM ref_table WHERE key_column=expr;
  
  SELECT * FROM ref_table,other_table
    WHERE ref_table.key_column=other_table.column;
  
  SELECT * FROM ref_table,other_table
    WHERE ref_table.key_column_part1=other_table.column
    AND ref_table.key_column_part2=1;
    
  --上述查询是ref类型的 
  

  explain SELECT * from table_0, table_1 where table_0.v_0 = table_1.v_1
  --表结构及包含的数据同eq_ref时举的例子
  

  id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	extra
  1	simple	table_0	null	index	v_0	v_0	5	null	2	100	Using where; Using index
  1	simple	table_1	null	ref	v_1	v_1	5	szn.table_0.id+0	1	100	Using index

• fulltext：使用全文索引

• ref_or_null：类似于ref，但是增加了搜索行是否为空的条件

  SELECT * FROM ref_table
    WHERE key_column=expr OR key_column IS NULL;
  
  --上述查询是ref_or_null类型的 
  

• index_merge：索引合并优化被使用

• unique_subquery：在以下形式的in子查询中，unique_subquery代替了eq_ref

  value IN (SELECT primary_key FROM single_table WHERE some_expr)
  

• index_subquery：类似于unique_subquery，但是作用的index不是唯一索引

  value IN (SELECT key_column FROM single_table WHERE some_expr)
  

• range：使用索引来检索一个范围的行，支持的操作符：=, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, LIKE, or IN()

  SELECT * FROM tbl_name
    WHERE key_column = 10;
  
  SELECT * FROM tbl_name
    WHERE key_column BETWEEN 10 and 20;
  
  SELECT * FROM tbl_name
    WHERE key_column IN (10,20,30);
  
  SELECT * FROM tbl_name
    WHERE key_part1 = 10 AND key_part2 IN (10,20,30);
  

• index：将扫描索引树，有二种情况：

  • 如果查询的索引是覆盖索引，则只有索引树被扫描(速度很快)。Extra列显示using index。
  • 以索引顺序，通过读取的索引，执行全表扫描来寻找数据。Extra列不会显示using index

• all：执行全表扫描。应尽量避免这种类型

key

• mysql实际决定使用的key
• 有可能不会出现在possible_keys的列表中

ref

​ ref指明了和key比较的是那一列或者常量

create table table_0(id_0 bigint primary key, v_0 int, index(v_0));
insert into table_0(id_0, v_0) values (0, 1), (1, 2);

explain SELECT * from table_0 where table_0.id_0 = 1
--ref列，值为 const

explain SELECT * from table_0 where id_0 = v_0
--ref列，值为 v_0

rows

• mysql认为执行查询必须检查的行数
• 对于Innodb，这个值是一个估算值，并不准确

filtered

• 根据查询条件，未过滤的行数的百分比
• 100表示未进行过滤
• 此值是一个预估值