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你能帮我索引我的表吗？

问题是我索引了我的表，但我的解释中仍然有“全表扫描”

这是我的（工作）查询，但在大表上它可能很慢，我不知道如何更改它

EXPLAIN select * from  stats_clicked s
join visitor v on s.visitor_id=v.id

附言。index3 - 当访问者 = 1 刷新 id = 5 的页面时，我不想多次取值 (1,5)

CREATE TABLE `visitor` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `visited_time` int(11) DEFAULT NULL,
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `stats_clicked` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `visitor_id` int(11) NOT NULL,
   `page_clicked_id` int(11) DEFAULT NULL,
   PRIMARY KEY (`id`),
   UNIQUE KEY `index3` (`visitor_id`,`page_clicked_id`),
   KEY `index1` (`visitor_id`)
 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;


insert into visitor (`visited_time`) values
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944);

insert into `stats_clicked` ( `visitor_id`,`page_clicked_id`) values
(1,47),(2,24),(3,83),(3,8),(3,85),(3,88),(4,57),
(5,2),(6,1),(7,28),(8,83),(9,11),(9,16),(9,1),(10,17),
(11,70),(12,73),(13,97),(14,57),(15,30),(15,2),(15,22);
4

2 回答 2

0

如果我执行你上面所做的，我会得到

EXPLAIN select * from  stats_clicked s 
join visitor v on s.visitor_id=v.id 
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+
| id | select_type | table | type | possible_keys | key    | key_len | ref                | rows | Extra       |
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+
|  1 | SIMPLE      | v     | ALL  | PRIMARY       | NULL   | NULL    | NULL               |   15 | NULL        |
|  1 | SIMPLE      | s     | ref  | index3,index1 | index3 | 4       | so_gibberish2.v.id |    1 | Using index |
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+

但是，如果我截断然后执行大量数据的以下加载（最终超过 100K 行）：

truncate table visitor;

insert into visitor (`visited_time`) values
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944);

insert into visitor (`visited_time`) values
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),


insert into visitor (`visited_time`) values
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946);

insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;

select count(*) from visitor;
-- 104448 rows

这导致不是表扫描：

EXPLAIN select * from  stats_clicked s 
join visitor v on s.visitor_id=v.id; 

+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+
|  1 | SIMPLE      | s     | index  | index3,index1 | index3  | 9       | NULL                       |   22 | Using index |
|  1 | SIMPLE      | v     | eq_ref | PRIMARY       | PRIMARY | 4       | so_gibberish2.s.visitor_id |    1 | NULL        |
+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+

原因在 Manual Page How MySQL Uses Indexes中列出：

对于小表或报表查询处理大部分或所有行的大表的查询，索引不太重要。当查询需要访问大部分行时，顺序读取比通过索引更快。即使查询不需要所有行，顺序读取也可以最大限度地减少磁盘寻道。

原因如上。在您的问题示例中，您的行数太少，无法使用索引。因此，数据库引擎选择了其所谓（并且可能）更快的方式，即不使用小表上的索引。

于 2016-06-29T19:35:01.653 回答
0

stats_clicked不需要id。事实上，它的存在减慢了对该表的所有可能查询。 更多细节。

于 2016-07-02T02:32:31.597 回答