MySQL Index Condition Pushdown(ICP)优化

mysql> desc employees.employees;
+------------+---------------+------+-----+---------+-------+
| Field      | Type          | Null | Key | Default | Extra |
+------------+---------------+------+-----+---------+-------+
| emp_no     | int(11)       | NO   | PRI | NULL    |       |
| birth_date | date          | NO   |     | NULL    |       |
| first_name | varchar(14)   | NO   |     | NULL    |       |
| last_name  | varchar(16)   | NO   |     | NULL    |       |
| gender     | enum(‘M‘,‘F‘) | NO   |     | NULL    |       |
| hire_date  | date          | NO   |     | NULL    |       |
+------------+---------------+------+-----+---------+-------+
6 rows in set (0.09 sec)

mysql> alter table employees add index idx_fn_ln (first_name,last_name );//添加联合索引
Query OK, 0 rows affected (3.98 sec)
Records: 0  Duplicates: 0  Warnings: 0

Index Condition Pushdown打开的情况下

mysql> explain select * from employees where first_name =‘Mary‘ and last_name like ‘%man‘;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-----------------------+
| id | select_type | table     | type | possible_keys | key       | key_len | ref   | rows | Extra                 |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | employees | ref  | idx_fn_ln     | idx_fn_ln | 16      | const |  224 | Using index condition |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

关闭的情况下：

mysql> set optimizer_switch = ‘index_condition_pushdown=off‘; //关闭index condition pushdown
Query OK, 0 rows affected (0.02 sec)

mysql> explain select * from employees where first_name =‘Mary‘ and last_name like ‘%man‘;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------------+
| id | select_type | table     | type | possible_keys | key       | key_len | ref   | rows | Extra       |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------------+
|  1 | SIMPLE      | employees | ref  | idx_fn_ln     | idx_fn_ln | 16      | const |  224 | Using where |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------------+
1 row in set (0.00 sec)

看where语句中包含上述联合索引，并且包含一个非索引列：

mysql> set optimizer_switch=‘index_condition_pushdown=on‘;
Query OK, 0 rows affected (0.00 sec)

mysql> explain select * from employees where first_name =‘Mary‘ and last_name =‘%man‘ and gender =‘M‘;
+----+-------------+-----------+------+---------------+-----------+---------+-------------+------+------------------------------------+
| id | select_type | table     | type | possible_keys | key       | key_len | ref         | rows | Extra                              |
+----+-------------+-----------+------+---------------+-----------+---------+-------------+------+------------------------------------+
|  1 | SIMPLE      | employees | ref  | idx_fn_ln     | idx_fn_ln | 34      | const,const |    1 | Using index condition; Using where |
+----+-------------+-----------+------+---------------+-----------+---------+-------------+------+------------------------------------+
1 row in set (0.01 sec)

  同样有using index condition,不过index过滤后，Server还要根据gender列来判断一下 storage engine返回的值；

   ICP只能用于二级索引，不能用于主索引。

   也不是全部where条件都可以用ICP筛选，如果某where条件的字段不在索引中，当然还是要读取整条记录做筛选，在这种情况下，仍然要到server端做where筛选。

   ICP的加速效果取决于在存储引擎内通过ICP筛选掉的数据的比例。

   本文使用employees数据库表和数据在这里************下载，该库功能数据齐全，employees_db-full-1.0.6.tar.bz2；

MySQL Index Condition Pushdown(ICP)优化

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