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ICP 测试
首先，咱们来看一下打开 ICP 与关闭 ICP 之间的性能区别，以下是测试过程：
准备数据：
create table icp(id int, age int, name varchar(30), memo varchar(600)) engine=innodb; alter table icp add index aind(age, name, memo); --let $i= 100000 while ($i)
{ --eval insert into icp values($i, 1, 'a$i', repeat('a$i', 100)) --dec $i }PS: MySQL 有一个叫profile的东东，可以用来监视 SQL 语句在各个阶段的执行情况，咱们可以使用这个工具来观察 SQL 语句在各个阶段的运行情况，关于 profile 的详细说明可以参考官方文档。
打开 ICP 的性能测试：
set profiling=on;
set optimizer_switch="index_condition_pushdown=on”; （default enabled）
mysql> select * from icp where age = 999 and name like '%999%';
+------+------+------+------+
| id | age | name | memo |
+------+------+------+------+
| NULL | 999 | 999 | 999 |
+------+------+------+------+
1 row in set (0.00 sec)
mysql> explain select * from icp where age = 999 and name like '%999%';
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------+
| 1 | SIMPLE | icp | ref | aind | aind | 5 | const | 1 | Using index condition |
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
mysql> show profiles;
+----------+------------+-----------------------------------------------------------------+
| Query_ID | Duration | Query |
+----------+------------+-----------------------------------------------------------------+
| 1 | 0.00043550 | select * from icp where age = 999 and name like '%999%' |
| 2 | 0.00043250 | explain select * from icp where age = 999 and name like '%999%' |
+----------+------------+-----------------------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
mysql> show profile cpu,block io for query 2;
+----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| starting | 0.000084 | 0.000000 | 0.000000 | 0 | 0 |
| checking permissions | 0.000011 | 0.000000 | 0.000000 | 0 | 0 |
| Opening tables | 0.000064 | 0.000000 | 0.000000 | 0 | 0 |
| init | 0.000046 | 0.000000 | 0.000000 | 0 | 0 |
| System lock | 0.000010 | 0.000000 | 0.000000 | 0 | 0 |
| optimizing | 0.000020 | 0.000000 | 0.000000 | 0 | 0 |
| statistics | 0.000082 | 0.000000 | 0.000000 | 0 | 0 |
| preparing | 0.000022 | 0.000000 | 0.000000 | 0 | 0 |
| explaining | 0.000021 | 0.000000 | 0.000000 | 0 | 0 |
| query end | 0.000008 | 0.000000 | 0.000000 | 0 | 0 |
| closing tables | 0.000022 | 0.000000 | 0.000000 | 0 | 0 |
| freeing items | 0.000031 | 0.000000 | 0.000000 | 0 | 0 |
| cleaning up | 0.000013 | 0.000000 | 0.000000 | 0 | 0 |
+----------------------+----------+----------+------------+--------------+---------------+
13 rows in set, 1 warning (0.00 sec)
mysql> show session status like '%handler%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 2 |
| Handler_delete | 0 |
| Handler_discover | 0 |
| Handler_external_lock | 4 |
| Handler_mrr_init | 0 |
| Handler_prepare | 0 |
| Handler_read_first | 0 |
| Handler_read_key | 1 |
| Handler_read_last | 0 |
| Handler_read_next | 1 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 42 |
| Handler_rollback | 0 |
| Handler_savepoint | 0 |
| Handler_savepoint_rollback | 0 |
| Handler_update | 0 |
| Handler_write | 39 |
+----------------------------+-------+
18 rows in set (0.00 sec)关闭 ICP 的性能测试：
mysql> set optimizer_switch="index_condition_pushdown=off”;
mysql> select * from icp where age = 1 and memo like '%9999%';
mysql> select * from icp where age = 999 and name like '%999%';
+------+------+------+------+
| id | age | name | memo |
+------+------+------+------+
| NULL | 999 | 999 | 999 |
+------+------+------+------+
1 row in set (0.00 sec)
mysql> explain select * from icp where age = 999 and name like '%999%';
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| 1 | SIMPLE | icp | ref | aind | aind | 5 | const | 1 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)
mysql> show profiles;
+----------+------------+-----------------------------------------------------------------+
| Query_ID | Duration | Query |
+----------+------------+-----------------------------------------------------------------+
| 1 | 0.00043550 | select * from icp where age = 999 and name like '%999%' |
| 2 | 0.00043250 | explain select * from icp where age = 999 and name like '%999%' |
| 3 | 0.00081350 | show session status like '%handler%' |
| 4 | 0.00010350 | set optimizer_switch="index_condition_pushdown=off" |
| 5 | 0.00036525 | select * from icp where age = 999 and name like '%999%' |
| 6 | 0.00032950 | explain select * from icp where age = 999 and name like '%999%' |
+----------+------------+-----------------------------------------------------------------+
6 rows in set, 1 warning (0.00 sec)
mysql> show profile cpu,block io for query 5;
+----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| starting | 0.000068 | 0.000000 | 0.000000 | 0 | 0 |
| checking permissions | 0.000007 | 0.000000 | 0.000000 | 0 | 0 |
| Opening tables | 0.000020 | 0.000000 | 0.000000 | 0 | 0 |
| init | 0.000032 | 0.000000 | 0.000000 | 0 | 0 |
| System lock | 0.000010 | 0.000000 | 0.000000 | 0 | 0 |
| optimizing | 0.000015 | 0.000000 | 0.000000 | 0 | 0 |
| statistics | 0.000088 | 0.000000 | 0.000000 | 0 | 0 |
| preparing | 0.000017 | 0.000000 | 0.000000 | 0 | 0 |
| executing | 0.000003 | 0.000000 | 0.000000 | 0 | 0 |
| Sending data | 0.000049 | 0.000000 | 0.000000 | 0 | 0 |
| end | 0.000005 | 0.000000 | 0.000000 | 0 | 0 |
| query end | 0.000007 | 0.000000 | 0.000000 | 0 | 0 |
| closing tables | 0.000008 | 0.000000 | 0.000000 | 0 | 0 |
| freeing items | 0.000024 | 0.000000 | 0.000000 | 0 | 0 |
| cleaning up | 0.000014 | 0.000000 | 0.000000 | 0 | 0 |
+----------------------+----------+----------+------------+--------------+---------------+
15 rows in set, 1 warning (0.00 sec)
mysql> show session status like '%handler%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 4 |
| Handler_delete | 0 |
| Handler_discover | 0 |
| Handler_external_lock | 8 |
| Handler_mrr_init | 0 |
| Handler_prepare | 0 |
| Handler_read_first | 0 |
| Handler_read_key | 2 |
| Handler_read_last | 0 |
| Handler_read_next | 2 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 58 |
| Handler_rollback | 0 |
| Handler_savepoint | 0 |
| Handler_savepoint_rollback | 0 |
| Handler_update | 0 |
| Handler_write | 54 |
+----------------------------+-------+
18 rows in set (0.00 sec)测试结论：由以上测试情况可以看到，在二级索引是复合索引且前面的条件过滤性较低的情况下，打开 ICP 可以有效的降低 server 层和 engine 层之间交互的次数，从而有效的降低在运行时间。
ICP 原理
5.6 之前，在 SQL 语句的执行过程中，server 层通过 engine 的 api 获取数据，然后再进行 where_cond 的判断（具体判断逻辑在: evaluate_join_record），每一条数据都需要从engine层返回server层做判断。我们回顾一下上面把 ICP 关掉的测试，可以看到 Handler_read_next 的值陡增，其原因是第 1 个字段区分度不高，且 memo 字段无法使用索引，造成了类似 index 扫描的的情况，性能较低。
5.6 之后，在利用索引扫描的过程中，如果发现 where_cond 中含有这个 index 相关的条件，则将此条件记录在 handler 接口中，在索引扫描的过程中，只有满足索引与handler接口的条件时，才会返回到 server 层做进一步的处理，在前缀索引区分度不够，其它字段区分度高的情况下可以有效的减少 server & engine之间的开销，提升查询性能。
ICP 源码实现
我们在上小节提到，index condition down 所用的条件是记在handler接口中的，咱们分析一下“记录”的过程是如何实现的。
首先，优化器计算代价后会生成一个 JOIN_TAB 的左支树，每一个 JOIN_TAB 包含相关表的指针、表的读取方式、访问表所包含的索引等信息，优化器会在make_join_readinfo 中对JOIN_TAB中表的访问方式进行相应的修正，并进一步将 where cond 中和索引相关的条件记录到 table 的句柄中，堆栈如下：
#0 make_cond_for_index (cond=0x2b69680179e8, table=0x2b6968012100, keyno=0, other_tbls_ok=true)
#1 in push_index_cond (tab=0x2b696802aa48, keyno=0, other_tbls_ok=true, trace_obj=0x2b696413ec30)
#2 in make_join_readinfo (join=0x2b6968017db0, options=0, no_jbuf_after=4294967295)
#3 in JOIN::optimize (this=0x2b6968017db0)
#4 in mysql_execute_select (thd=0x3176760, select_lex=0x3179470, free_join=true)其次， make_cond_for_index 是一个递归的过程，对 where_cond中的每一个条件进行判断，对满足条件的 cond 重新组合成一个新的cond，最后将新的 cond 挂在table->file 下面（table->file 指的是操作物理表的接口函数，此变量为thd下私有的，不共享，共享的是tab->table->s），详细参考make_cond_for_index 的详细实现，设置的堆栈如下：
#0 ha_innobase::idx_cond_push (this=0x2b696800e810, keyno=0, idx_cond=0x2b69680179e8)
#1 0x0000000000a60a55 in push_index_cond (tab=0x2b696802aa48, keyno=0, other_tbls_ok=true, trace_obj=0x2b696413ec30)
#2 0x0000000000a6362f in make_join_readinfo (join=0x2b6968017db0, options=0, no_jbuf_after=4294967295)
#3 0x0000000000d9b8bd in JOIN::optimize (this=0x2b6968017db0 #4 0x0000000000a5b9ae in mysql_execute_select (thd=0x3176760, select_lex=0x3179470, free_join=true)再次，server 层根据生成的 JOIN_TAB 读取engine层的内容，在engine读取的时候，会进行index_condition_pushdown的调用，即 ICP 的调用，堆栈如下：
#0 Item_func_like::val_int (this=0x2b6978005a28)
#1 0x0000000001187b66 in innobase_index_cond (file=0x2b696800e810)
#2 0x0000000001393566 in row_search_idx_cond_check (mysql_rec=0x2b69680129f0 <incomplete sequence="" \361>, prebuilt=0x2b69680130f8, rec=0x2b692b56e4cf "\200", offsets=0x2b697008d450)
#3 0x0000000001397e2b in row_search_for_mysql (buf=0x2b69680129f0 <incomplete sequence="" \361>, mode=2, prebuilt=0x2b69680130f8, match_mode=1, direction=0)
#4 0x00000000011696b9 in ha_innobase::index_read (this=0x2b696800e810, buf=0x2b69680129f0 <incomplete sequence="" \361>, key_ptr=0x2b697800a660 "", key_len=5, find_flag=HA_READ_KEY_EXACT)
#5 0x00000000006ecc58 in handler::index_read_map (this=0x2b696800e810, buf=0x2b69680129f0 <incomplete sequence="" \361>, key=0x2b697800a660 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT)
#6 0x00000000006d6bb4 in handler::ha_index_read_map (this=0x2b696800e810, buf=0x2b69680129f0 <incomplete sequence="" \361>, key=0x2b697800a660 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT)
#7 0x00000000009a1870 in join_read_always_key (tab=0x2b697800a1b8)
#8 0x000000000099d480 in sub_select (join=0x2b6978005df0, join_tab=0x2b697800a1b8, end_of_records=false)
#9 0x000000000099c6c0 in do_select (join=0x2b6978005df0)
#10 0x00000000009980a4 in JOIN::exec (this=0x2b6978005df0)
#11 0x0000000000a5bac0 in mysql_execute_select (thd=0x32801a0, select_lex=0x3282eb0, free_join=true)可见在 ICP 的判断是调用相关item的函数的，虽然同是调用 server 层的函数，但是没有 ICP 的调用需要根据主建找到记录，然后再匹配，而有了 ICP 可以省略一次主键查找数据的过程，进而提升效率。
ICP 使用限制及问题
只支持 select 语句；
5.6 中只支持 MyISAM 与 InnoDB 引擎;
ICP的优化策略可用于range、ref、eq_ref、ref_or_null 类型的访问数据方法；
不支持主建索引的 ICP；
当 SQL 使用覆盖索引时但只检索部分数据时，ICP 无法使用，详细的分析可以参考 bug#68554 中 Olav Sandst?的分析，代码实现部分可以参考 make_join_readinfo；
在查询的时候即使正确的使用索引的前Ｎ个字段（即遵循前缀索引的原则），还是会用到 ICP，无故的多了 ICP 相关的判断，这应该是一个退化的问题，例：
mysql> explain select * from icp where age = 999 and name like '999%'; +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | icp | range | aind | aind | 98 | NULL | 1 | Using index condition | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
PS: engine condition pushdown 是 NDB 使用的，其它引擎不支持。
补充：
如：
root@read 02:28:07>show status like ‘Handler_read%’;
+———————–+——-+
| Variable_name | Value |
+———————–+——-+
| Handler_read_first | 0 |
| Handler_read_key | 0 |
| Handler_read_next | 0 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 61 |
+———————–+——-+
6 rows in set (0.41 sec)Handler_read_first 代表读取索引头的次数，如果这个值很高，说明全索引扫描很多。
Handler_read_key代表一个索引被使用的次数，如果我们新增加一个索引，可以查看Handler_read_key是否有增加，如果有增加，说明sql用到索引。
Handler_read_next 代表读取索引的下列，一般发生range scan。
Handler_read_prev 代表读取索引的上列，一般发生在ORDER BY … DESC。
Handler_read_rnd 代表在固定位置读取行，如果这个值很高，说明对大量结果集进行了排序、进行了全表扫描、关联查询没有用到合适的KEY。
Handler_read_rnd_next 代表进行了很多表扫描，查询性能低下。
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