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自MySQL 5.6開(kāi)始，在索引方面有了一些改進(jìn)，比如索引條件下推(Index condition pushdown,ICP),嚴(yán)格來(lái)說(shuō)屬于優(yōu)化器層面的改進(jìn)。
如果簡(jiǎn)單來(lái)理解，就是優(yōu)化器會(huì)盡可能的把index condition的處理從Server層下推到存儲(chǔ)引擎層。舉一個(gè)例子，有一個(gè)表中含有組合索引idx_cols包含(c1,c2,…,cn)n個(gè)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(gè)上索引都無(wú)法用來(lái)提取和過(guò)濾數(shù)據(jù),而ICP就是把這個(gè)事情優(yōu)化一下。
我們?cè)贛ySQL 5.6的環(huán)境中來(lái)簡(jiǎn)單測(cè)試一下。
我們創(chuàng)建表emp，含有一個(gè)主鍵，一個(gè)組合索引來(lái)說(shuō)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(dāng)然我也隨機(jī)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(kù)參數(shù)中有一個(gè)優(yōu)化器參數(shù)optimizer_switch來(lái)統(tǒng)一管理，我想這也是MySQL優(yōu)化器離我們最貼近的時(shí)候了?？梢允褂萌缦碌姆绞絹?lái)查看。
- show variables like 'optimizer_switch';
當(dāng)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(gè)語(yǔ)句來(lái)對(duì)比說(shuō)明一下，就通過(guò)執(zhí)行計(jì)劃來(lái)對(duì)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(kāi)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(xì)，會(huì)發(fā)現(xiàn)兩次的語(yǔ)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(lái)的語(yǔ)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(gè)地方就值得好好推敲了。
當(dāng)前題目：MySQL索引條件下推的簡(jiǎn)單測(cè)試
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