MySQL 5.6新特性 -- Index Condition Pushdown
阿新 • • 發佈：2017-11-06
部分 isa mysq 支持 分區表 explain int mission iss 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之後，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後，在執行計劃的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： 
開啟ICP之後，就變成： 
默認是開啟ICP的，手動開啟/關閉ICP：
開啟ICP之後，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.環境準備#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後進行測試
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後進行測試
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後的執行計劃：執行計劃中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後的執行計劃：執行計劃中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後的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後的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 | +----+-------------+-----------+------+---------------------+---------------------+---------+-------------+------+-----------------------+
MySQL 5.6新特性 -- Index Condition Pushdown