Index Condition Pushdown(ICP)是针对mysql使用索引从表中检索行数据时的一种优化方法。
原理:
在没有ICP特性之前,存储引擎根据索引去基表查找并将数据返回给mysql server,mysql server再根据where条件进行数据过滤。
有了ICP之后,在取出索引的同时,判断是否可以根据索引中的列进行where条件过滤,也就是将where的部分过滤操作放在了存储引擎层。这样就会减少上层sql层对记录的获取。
当sql使用覆盖索引时,不支持ICP优化方法。
ICP优化支持range、ref、eq_ref、ref_or_null类型的查询。查询优化器会给出相应的提示:Using index condition。当开启ICP后,在执行计划的extra列会显示:Using index condition。
ICP支持innodb、myisam表。对于innodb表,ICP只是用于辅助索引。
在5.6中,ICP不支持分区表。这个问题在mysql 5.7中得到解决。
优化器使用ICP时,server层将会把能够通过使用索引进行评估的where条件下推到storage engine层。数据访问和提取过程如下:
1) storage engine从索引中读取下一条索引元组。
2) storage engine使用索引元组评估下推的索引条件。如果没有满足where条件,storage engine将会处理下一条索引元组(回到上一步)。只有当索引元组满足下推的索引条件的时候,才会继续去基表中读取数据。
3) 如果满足下推的索引条件,storage engine通过索引元组定位基表的行和读取整行数据并返回给server层。
4) server层评估没有被下推到storage engine层的where条件,如果该行数据满足where条件则使用,否则丢弃。
没有ICP之前:
开启ICP之后,就变成:
默认是开启ICP的,手动开启/关闭ICP:
set
optimizer_switch =
'index_condition_pushdown=off'
;
set
optimizer_switch =
'index_condition_pushdown=on'
;
测试过程
1.环境准备
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#mysql 5.6.25 #关闭结果缓存 mysql> set global query_cache_size=0;
mysql> set query_cache_type= off ;
#查看表结构 mysql> show create table employees\G
*************************** 1. row *************************** Table : employees
Create Table : CREATE TABLE `employees` (
`emp_no` int (11) NOT NULL ,
`birth_date` date NOT NULL ,
`first_name` varchar (14) NOT NULL ,
`last_name` varchar (16) NOT NULL ,
`gender` enum( 'M' , 'F' ) NOT NULL ,
`hire_date` date NOT NULL ,
PRIMARY KEY (`emp_no`),
KEY `idx_first_last_name` (`first_name`,`last_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
mysql> |
2.开启ICP后进行测试
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mysql> set profiling = 1;
mysql> select * from employees where first_name= 'Anneke' and last_name like '%sig' ;
mysql> explain select * from employees where first_name= 'Anneke' and last_name like '%sig' ;
mysql> show profiles; mysql> show profile cpu,block io for query 1;
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3.关闭ICP后进行测试
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mysql> set optimizer_switch= 'index_condition_pushdown=off' ;
mysql> set profiling = 1;
mysql> select * from employees where first_name= 'Anneke' and last_name like '%sig' ;
mysql> explain select * from employees where first_name= 'Anneke' and last_name like '%sig' ;
mysql> show profiles; mysql> show profile cpu,block io for query 1;
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4.结果比较
开启ICP后的执行计划:执行计划中extra部分的内容是"using index condition"
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mysql> explain select * from employees where first_name= 'Anneke' and last_name like '%sig' ;
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | employees | ref | idx_first_last_name | idx_first_last_name | 44 | const | 224 | Using index condition |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-----------------------+
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关闭ICP后的执行计划:执行计划中extra部分的内容是"using where"
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mysql> explain select * from employees where first_name= 'Anneke' and last_name like '%sig' ;
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-------------+
| 1 | SIMPLE | employees | ref | idx_first_last_name | idx_first_last_name | 44 | const | 224 | Using where |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-------------+
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开启ICP后的profile内容:Sending data部分的值是0.000212s
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mysql> show profile cpu,block io for query 1;
+ ----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | + ----------------------+----------+----------+------------+--------------+---------------+
| starting | 0.000114 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000007 | 0.000000 | 0.000000 | 0 | 0 | | Opening tables | 0.000018 | 0.000000 | 0.000000 | 0 | 0 | | init | 0.000034 | 0.000000 | 0.000000 | 0 | 0 | | System lock | 0.000008 | 0.000000 | 0.000000 | 0 | 0 | | optimizing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | | statistics | 0.000383 | 0.000000 | 0.000000 | 0 | 0 |
| preparing | 0.000019 | 0.000000 | 0.000000 | 0 | 0 | | executing | 0.000002 | 0.000000 | 0.000000 | 0 | 0 | | Sending data | 0.000212 | 0.000000 | 0.000000 | 0 | 0 | | end | 0.000004 | 0.000000 | 0.000000 | 0 | 0 |
| query end | 0.000004 | 0.000000 | 0.000000 | 0 | 0 |
| closing tables | 0.000006 | 0.000000 | 0.000000 | 0 | 0 | | freeing items | 0.000020 | 0.000000 | 0.000000 | 0 | 0 | | cleaning up | 0.000011 | 0.000000 | 0.000000 | 0 | 0 | + ----------------------+----------+----------+------------+--------------+---------------+
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关闭ICP后的profile内容:Sending data部分的值是0.010990s
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mysql> show profile cpu,block io for query 1;
+ ----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | + ----------------------+----------+----------+------------+--------------+---------------+
| starting | 0.000165 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | | Opening tables | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | | init | 0.000039 | 0.000000 | 0.000000 | 0 | 0 | | System lock | 0.000008 | 0.000000 | 0.000000 | 0 | 0 | | optimizing | 0.000037 | 0.001000 | 0.000000 | 0 | 0 | | statistics | 0.000483 | 0.001000 | 0.000000 | 0 | 0 |
| preparing | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | | executing | 0.000002 | 0.000000 | 0.000000 | 0 | 0 | | Sending data | 0.010990 | 0.007999 | 0.002000 | 0 | 0 | | end | 0.000009 | 0.000000 | 0.000000 | 0 | 0 |
| query end | 0.000005 | 0.000000 | 0.000000 | 0 | 0 |
| closing tables | 0.000008 | 0.000000 | 0.000000 | 0 | 0 | | freeing items | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | | cleaning up | 0.000014 | 0.000000 | 0.000000 | 0 | 0 | + ----------------------+----------+----------+------------+--------------+---------------+
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其它:
当sql使用覆盖索引时,不支持ICP优化方法
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mysql> explain select first_name,last_name from employees where first_name= 'Anneke' and last_name= 'Porenta' ;
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+--------------------------+
| 1 | SIMPLE | employees | ref | idx_first_last_name | idx_first_last_name | 94 | const,const | 1 | Using where ; Using index |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+--------------------------+
mysql> explain select * from employees where first_name= 'Anneke' and last_name= 'Porenta' ;
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+-----------------------+
| 1 | SIMPLE | employees | ref | idx_first_last_name | idx_first_last_name | 94 | const,const | 1 | Using index condition |
+ ----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+-----------------------+
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