MySQL 5.6新特性 -- Index Condition Pushdown

Index Condition Pushdown(ICP)是針對mysql使用索引從表中檢索行數據時的一種優化方法。

 

在沒有ICP特性之前，存儲引擎根據索引去基表查找並將數據返回給mysql server，mysql server再根據where條件進行數據過濾。

有了ICP之后，在取出索引的同時，判斷是否可以根據索引中的列進行where條件過濾，也就是將where的部分過濾操作放在了存儲引擎層。這樣就會減少上層sql層對記錄的獲取。

 

ICP優化支持range、ref、eq_ref、ref_or_null類型的查詢。查詢優化器會給出相應的提示：Using index condition。當開啟ICP后，在執行計划的extra列會顯示：Using index condition。

 

ICP支持innodb、myisam表。對於innodb表，ICP只是用於輔助索引。

 

在5.6中，ICP不支持分區表。這個問題在mysql 5.7中得到解決。

 

優化器使用ICP時，server層將會把能夠通過使用索引進行評估的where條件下推到storage engine層。數據訪問和提取過程如下：

1) storage engine從索引中讀取下一條索引元組。

2) storage engine使用索引元組評估下推的索引條件。如果沒有滿足where條件，storage engine將會處理下一條索引元組（回到上一步）。只有當索引元組滿足下推的索引條件的時候，才會繼續去基表中讀取數據。

3) 如果滿足下推的索引條件，storage engine通過索引元組定位基表的行和讀取整行數據並返回給server層。

4) server層評估沒有被下推到storage engine層的where條件，如果該行數據滿足where條件則使用，否則丟棄。

 

沒有ICP之前：

 

開啟ICP之后，就變成：

默認是開啟ICP的，手動開啟/關閉ICP： 

set optimizer_switch = 'index_condition_pushdown=off';
set optimizer_switch = 'index_condition_pushdown=on';

　　

 

 測試過程

 1.環境准備

#mysql 5.6.25
#關閉結果緩存
mysql> set global query_cache_size=0; 
mysql> set query_cache_type=off; 

#查看表結構
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.開啟ICP后進行測試

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.關閉ICP后進行測試

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.結果比較

開啟ICP后的執行計划：執行計划中extra部分的內容是"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 |
+----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-----------------------+

關閉ICP后的執行計划：執行計划中extra部分的內容是"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 |
+----+-------------+-----------+------+---------------------+---------------------+---------+-------+------+-------------+

　

開啟ICP后的profile內容：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 |
+----------------------+----------+----------+------------+--------------+---------------+

關閉ICP后的profile內容：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：

當sql使用覆蓋索引時，不支持ICP優化方法

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 |
+----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+-----------------------+