Mysql不在带偏移量的查询中使用索引

Question

我有一个包含大量行的简单数据库。

CREATE TABLE `tbl`(
    `id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    `name` NCHAR(50) NOT NULL
    ... other fields ...
  ) Engine=innodb;

我想做分页模仿。而且我知道经典的偏移是缓慢的操作。这就是为什么我尝试了这个how to automatically detect your USB drive letter

的技巧

我的查询版本是

SELECT * FROM `tbl` JOIN (SELECT id FROM `tbl` ORDER BY id LIMIT 1000000, 100) as b on `b`.`id` = `tbl`.`id`;

但诀窍无效。查询工作速度非常慢，因为mysql仍然会读取所有1000000行。

甚至分离的内部查询仅与主键有效，但不使用b树。

SELECT id FROM `tbl` ORDER BY id LIMIT 1000000, 100

那么，为什么当这个查询被它完全覆盖时，mysql引擎不使用主索引？

此外，我无法使用where id > SOME_NUMBER条件，因为我的表格非常稀疏，并且我不知道用户请求的随机页面的边界ID。

Answer 1

考虑以下内容，我强制将2.2M行放入表中

模式

create table tbl
(   id int auto_increment primary key,
    thing int not null
)engine=MyISAM;

insert tbl(thing) values (7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7),(7);

insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;
insert into tbl(thing) select thing from tbl;

计数

select count(*) as theCount,max(id) as theMax,min(id) as thMin from tbl;
+----------+---------+-------+
| theCount | theMax  | thMin |
+----------+---------+-------+
|  2228224 | 2228224 |     1 |
+----------+---------+-------+

查询A（您的，使用派生表）

explain 
SELECT *  
FROM `tbl` 
JOIN 
(SELECT id FROM `tbl` ORDER BY id LIMIT 1000000, 100) as b 
on `b`.`id` = `tbl`.`id`;
+----+-------------+------------+--------+---------------+---------+---------+------+---------+-------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows    | Extra       |
+----+-------------+------------+--------+---------------+---------+---------+------+---------+-------------+
|  1 | PRIMARY     | <derived2> | ALL    | NULL          | NULL    | NULL    | NULL | 1000100 | NULL        |
|  1 | PRIMARY     | tbl        | eq_ref | PRIMARY       | PRIMARY | 4       | b.id |       1 | NULL        |
|  2 | DERIVED     | tbl        | index  | NULL          | PRIMARY | 4       | NULL | 2228224 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+------+---------+-------------+

查询B（不是派生表）

explain 
SELECT t1.*  
FROM tbl t1 
JOIN tbl t2 
on t2.id = t1.id 
where t2.id>1000000 
limit 100 
+----+-------------+-------+--------+---------------+---------+---------+--------------------+---------+--------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                | rows    | Extra                    |
+----+-------------+-------+--------+---------------+---------+---------+--------------------+---------+--------------------------+
|  1 | SIMPLE      | t2    | range  | PRIMARY       | PRIMARY | 4       | NULL               | 1195836 | Using where; Using index |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | so_gibberish.t2.id |       1 | NULL                     |
+----+-------------+-------+--------+---------------+---------+---------+--------------------+---------+--------------------------+

对于那些不熟悉使用Explain的人，请参阅我写的here