MySql JOIN性能随分组而下降 [英] MySql JOIN performance go down with group by

问题描述

我有这些表:

table "f" (26000 record)
+------------------+------------------+------+-----+---------+-------+
| Field            | Type             | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| idFascicolo      | int(11)          | NO   | PRI |         |       |
| oggetto          | varchar          | NO   |index|         |       |
+------------------+------------------+------+-----+---------+-------+

table "r" (22000 record)
+------------------+------------------+------+-----+---------+-------+
| Field            | Type             | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| idRichiedente    | int(11)          | NO   | PRI |         |       |
| name             | varchar          | NO   |index|         |       |
+------------------+------------------+------+-----+---------+-------+

table "fr" (32000 record)
+------------------+------------------+------+-----+---------+-------+
| Field            | Type             | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| id               | int(11)          | NO   | PRI |         |       |
| idFascicolo      | int(11)          | NO   |index|         |  FK   |
| idRichiedente    | int(11)          | NO   |index|         |  FK   |
+------------------+------------------+------+-----+---------+-------+

这是我的选择

SELECT
f.idFascicolo,
f.oggetto,
r.richiedente
FROM fr
JOIN f ON (f.idFascicolo=fr.idFascicolo)
JOIN r ON (r.idRichiedente=fr.idRichiedente)
WHERE r.name LIKE '%string%'

在结果中，我希望每个f.idFascicolo仅看到1行(对于同一f.idFascicolo，我应该有"Rossi Mario"和"Rossi Marco")，我的新选择是:

in the result, I would like to see only 1 row per f.idFascicolo (I should have "Rossi Mario" and "Rossi Marco" for the same f.idFascicolo) , the my new select is:

SELECT
f.idFascicolo,
f.oggetto,
r.richiedente
FROM fr
JOIN f ON (f.idFascicolo=fr.idFascicolo)
JOIN r ON (r.idRichiedente=fr.idRichiedente)
WHERE r.name LIKE '%string%'
GROUP BY f.idFascicolo

此处，性能是从PhpMyAdmin中读取的:

here, the performance read from PhpMyAdmin:

0.0057 seconds: .. WHERE r.name LIKE '%string%'
0.0527 seconds: .. WHERE r.name LIKE '%string%' GROUP BY f.idFascicolo
0.0036 seconds: .. WHERE r.name LIKE 'string%' GROUP BY f.idFascicolo

我不明白慢查询的问题是GROUP BY还是LIKE'％string％'(我需要'％string％'..我找不到全文索引和MATCH的等效解决方案.反对)

I don't understand if the problem of the slow query is GROUP BY or LIKE '%string%'(i need '%string%' .. I can't find an equivalent solution with fulltext index and MATCH .. AGAINST)

这是解释:

+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| id   | select type | table | type | possible keys           |  key          | key_len | ref                  | rows      | Extra                                       |
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| 1    | simple      | r     | ALL  | PRIMARY                 | NULL          | NULL    | NULL                 | 20925     |Using where; Using temporary; Using filesort | 
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+ 
| 1    | simple      | fr    | ref  |idFascicolo,idRichiedente| idRichiedente | 4       | db.r.idRichiedente   | 1         |                                             |  
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+
| 1    | simple      | f     |eq_ref|PRIMARY                  | PRIMARY       | 4       | db.fr.idFascicolo    | 1         |                                             |  
+------+-------------+-------+------+-------------------------+---------------+---------+----------------------+-----------+---------------------------------------------+

推荐答案

您有两个潜在的性能问题.首先是GROUP BY.这需要对数据进行排序，因此它必须读取所有数据并做很多工作.

You have two potential performance issues. First is the GROUP BY. This requires sorting the data, so it has to read all the data and do a lot of work.

第二个是LIKE.两者之间有根本的区别:

The second is the LIKE. There is a fundamental difference between:

WHERE r.name LIKE '%string%' 

和

WHERE r.name LIKE 'string%' 

第二个可以使用r(name)上的索引，因为like模式不是不是以模式开头.

The second can use an index on r(name), because the like pattern does not start with a pattern.

我不确定您的实际问题是什么.我不建议您按自己的方式使用GROUP BY －因为您在SELECT中具有未聚合的列.

I am not sure what your actual question is. I don't recommend doing using GROUP BY the way you are using it -- because you have unaggregated columns in the SELECT.

这篇关于MySql JOIN性能随分组而下降的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！