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自MySQL 5.6開始，在索引方面有了一些改進，比如索引條件下推(Index condition pushdown,ICP),嚴格來說屬于優(yōu)化器層面的改進。
如果簡單來理解，就是優(yōu)化器會盡可能的把index condition的處理從Server層下推到存儲引擎層。舉一個例子，有一個表中含有組合索引idx_cols包含(c1,c2,…,cn)n個列，如果在c1上存在范圍掃描的where條件，那么剩余的c2,…,cn這n-1個上索引都無法用來提取和過濾數(shù)據(jù),而ICP就是把這個事情優(yōu)化一下。
我們在MySQL 5.6的環(huán)境中來簡單測試一下。
我們創(chuàng)建表emp，含有一個主鍵，一個組合索引來說明一下。
- create table emp(
- empno smallint(5) unsigned not null auto_increment,
- ename varchar(30) not null,
- deptno smallint(5) unsigned not null,
- job varchar(30) not null,
- primary key(empno),
- key idx_emp_info(deptno,ename)
- )engine=InnoDB charset=utf8;
當然我也隨機插入了幾條數(shù)據(jù)，意思一下。
- insert into emp values(1,'zhangsan',1,'CEO'),(2,'lisi',2,'CFO'),(3,'wangwu',3,'CTO'),(4,'jeanron100',3,'Enginer');
ICP的控制在數(shù)據(jù)庫參數(shù)中有一個優(yōu)化器參數(shù)optimizer_switch來統(tǒng)一管理，我想這也是MySQL優(yōu)化器離我們最貼近的時候了?？梢允褂萌缦碌姆绞絹聿榭?。
- show variables like 'optimizer_switch';
當然在5.6以前的版本中，你是看不到index condition pushdown這樣的字樣的。在5.6版本中查看到的結(jié)果如下：
# mysqladmin var|grep optimizer_switch optimizer_switch | index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,subquery_materialization_cost_based=on,use_index_extensions=on下面我們就用兩個語句來對比說明一下，就通過執(zhí)行計劃來對比。
- set optimizer_switch = "index_condition_pushdown=off"
- > explain select * from emp where deptno between 1 and 100 and ename ='jeanron100';
- +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
- | 1 | SIMPLE | emp | ALL | idx_emp_info | NULL | NULL | NULL | 4 | Using where |
- +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
而如果開啟，看看ICP是否啟用。
- set optimizer_switch = "index_condition_pushdown=on";> explain select * from emp where deptno between 10 and 3000 and ename ='jeanron100';
- +----+-------------+-------+-------+---------------+--------------+---------+------+------+-----------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+-------+---------------+--------------+---------+------+------+-----------------------+
- | 1 | SIMPLE | emp | range | idx_emp_info | idx_emp_info | 94 | NULL | 1 | Using index condition |
- +----+-------------+-------+-------+---------------+--------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)如果你觀察仔細，會發(fā)現(xiàn)兩次的語句還是不同的，那就是范圍掃描的范圍不同，如果還是用原來的語句，結(jié)果還是有一定的限制的。
- > explain select * from emp where deptno between 1 and 300 and ename ='jeanron100';
- +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
- | 1 | SIMPLE | emp | ALL | idx_emp_info | NULL | NULL | NULL | 4 | Using where |
- +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)這個地方就值得好好推敲了。
網(wǎng)頁標題：MySQL索引條件下推的簡單測試
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