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。
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。
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
#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
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;
3.关闭ICP后进行测试
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;
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"
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 |
+----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-----------------------+
关闭ICP后的执行计划：执行计划中extra部分的内容是"using where"
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 |
+----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-------------+
开启ICP后的profile内容：Sending data部分的值是0.000212s
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 |
+----------------------+----------+----------+------------+--------------+---------------+
关闭ICP后的profile内容：Sending data部分的值是0.010990s
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 |
+----------------------+----------+----------+------------+--------------+---------------+
其它：
当sql使用覆盖索引时，不支持ICP优化方法
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 |
+----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+-----------------------+