mysql查询运用select敕令,合营limit,offset参数能够读取指定局限的纪录。本文将引见mysql查询时,offset过大影响机能的缘由及优化要领。
预备测试数据表及数据
1.建立表
CREATE TABLE `member` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `name` varchar(10) NOT NULL COMMENT '姓名', `gender` tinyint(3) unsigned NOT NULL COMMENT '性别', PRIMARY KEY (`id`), KEY `gender` (`gender`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
2.插进去1000000条纪录
<?php $pdo = new PDO("mysql:host=localhost;dbname=user","root",'');for($i=0; $i<1000000; $i++){ $name = substr(md5(time().mt_rand(000,999)),0,10); $gender = mt_rand(1,2); $sqlstr = "insert into member(name,gender) values('".$name."','".$gender."')"; $stmt = $pdo->prepare($sqlstr); $stmt->execute();} ?>mysql> select count(*) from member; +----------+| count(*) | +----------+| 1000000 | +----------+1 row in set (0.23 sec)
3.当前数据库版本
mysql> select version(); +-----------+| version() | +-----------+| 5.6.24 | +-----------+1 row in set (0.01 sec)
剖析offset过大影响机能的缘由
1.offset较小的状况
mysql> select * from member where gender=1 limit 10,1; +----+------------+--------+| id | name | gender | +----+------------+--------+| 26 | 509e279687 | 1 | +----+------------+--------+1 row in set (0.00 sec)mysql> select * from member where gender=1 limit 100,1; +-----+------------+--------+| id | name | gender | +-----+------------+--------+| 211 | 07c4cbca3a | 1 | +-----+------------+--------+1 row in set (0.00 sec)mysql> select * from member where gender=1 limit 1000,1; +------+------------+--------+| id | name | gender | +------+------------+--------+| 1975 | e95b8b6ca1 | 1 | +------+------------+--------+1 row in set (0.00 sec)
当offset较小时,查询速率很快,效力较高。
2.offset较大的状况
mysql> select * from member where gender=1 limit 100000,1; +--------+------------+--------+| id | name | gender | +--------+------------+--------+| 199798 | 540db8c5bc | 1 | +--------+------------+--------+1 row in set (0.12 sec)mysql> select * from member where gender=1 limit 200000,1; +--------+------------+--------+| id | name | gender | +--------+------------+--------+| 399649 | 0b21fec4c6 | 1 | +--------+------------+--------+1 row in set (0.23 sec)mysql> select * from member where gender=1 limit 300000,1; +--------+------------+--------+| id | name | gender | +--------+------------+--------+| 599465 | f48375bdb8 | 1 | +--------+------------+--------+1 row in set (0.31 sec)
当offset很大时,会涌现效力题目,跟着offset的增大,实行效力下落。
剖析影响机能缘由
select * from member where gender=1 limit 300000,1;
由于数据表是InnoDB,依据InnoDB索引的构造,查询历程为:
经由过程二级索引查到主键值(找出一切gender=1的id)。
再依据查到的主键值经由过程主键索引找到响应的数据块(依据id找出对应的数据块内容)。
依据offset的值,查询300001次主键索引的数据,末了将之前的300000条抛弃,掏出末了1条。
不过既然二级索引已找到主键值,为何还须要先用主键索引找到数据块,再依据offset的值做偏移处置惩罚呢?
假如在找到主键索引后,先实行offset偏移处置惩罚,跳过300000条,再经由过程第300001条纪录的主键索引去读取数据块,如许就可以进步效力了。
假如我们只查询出主键,看看有什么差别
mysql> select id from member where gender=1 limit 300000,1; +--------+| id | +--------+| 599465 | +--------+1 row in set (0.09 sec)
很明显,假如只查询主键,实行效力对照查询悉数字段,有很大的提拔。
推想
只查询主键的状况
由于二级索引已找到主键值,而查询只须要读取主键,因而mysql会先实行offset偏移操纵,再依据背面的主键索引读取数据块。
须要查询一切字段的状况
由于二级索引只找到主键值,但其他字段的值须要读取数据块才猎取。因而mysql会先读出数据块内容,再实行offset偏移操纵,末了抛弃前面须要跳过的数据,返回背面的数据。
证明
InnoDB中有buffer pool,寄存近来接见过的数据页,包含数据页和索引页。
为了测试,先把mysql重启,重启后检察buffer pool的内容。
mysql> select index_name,count(*) from information_schema.INNODB_BUFFER_PAGE where INDEX_NAME in('primary','gender') and TABLE_NAME like '%member%' group by index_name; Empty set (0.04 sec)
能够看到,重启后,没有接见过任何的数据页。
查询一切字段,再检察buffer pool的内容
mysql> select * from member where gender=1 limit 300000,1; +--------+------------+--------+| id | name | gender | +--------+------------+--------+| 599465 | f48375bdb8 | 1 | +--------+------------+--------+1 row in set (0.38 sec)mysql> select index_name,count(*) from information_schema.INNODB_BUFFER_PAGE where INDEX_NAME in('primary','gender') and TABLE_NAME like '%member%' group by index_name; +------------+----------+| index_name | count(*) | +------------+----------+| gender | 261 || PRIMARY | 1385 | +------------+----------+2 rows in set (0.06 sec)
能够看出,此时buffer pool中关于member表有1385个数据页,261个索引页。
重启mysql清空buffer pool,继承测试只查询主键
mysql> select id from member where gender=1 limit 300000,1; +--------+| id | +--------+| 599465 | +--------+1 row in set (0.08 sec)mysql> select index_name,count(*) from information_schema.INNODB_BUFFER_PAGE where INDEX_NAME in('primary','gender') and TABLE_NAME like '%member%' group by index_name; +------------+----------+| index_name | count(*) | +------------+----------+| gender | 263 || PRIMARY | 13 | +------------+----------+2 rows in set (0.04 sec)
能够看出,此时buffer pool中关于member表只要13个数据页,263个索引页。因而减少了屡次经由过程主键索引接见数据块的I/O操纵,进步实行效力。
因而能够证明,mysql查询时,offset过大影响机能的缘由是屡次经由过程主键索引接见数据块的I/O操纵。(注重,只要InnoDB有这个题目,而MYISAM索引构造与InnoDB差别,二级索引都是直接指向数据块的,因而没有此题目 )。
InnoDB与MyISAM引擎索引构造对照图
优化要领
依据上面的剖析,我们晓得查询一切字段会致使主键索引屡次接见数据块形成的I/O操纵。
因而我们先查出偏移后的主键,再依据主键索引查询数据块的一切内容即可优化。
mysql> select a.* from member as a inner join (select id from member where gender=1 limit 300000,1) as b on a.id=b.id; +--------+------------+--------+| id | name | gender | +--------+------------+--------+| 599465 | f48375bdb8 | 1 | +--------+------------+--------+1 row in set (0.08 sec)
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