使用MySQL特定指数与轨道

我有这样的ActiveRecord查询使用MySQL特定指数与轨道

issue = Issue.find(id) 
issue.articles.includes(:category).merge(Category.where(permalink: perma))

而转换为MySQL查询

SELECT `articles`.`id` AS t0_r0, `articles`.`title` AS t0_r1, 
     `articles`.`hypertitle` AS t0_r2, `articles`.`html` AS t0_r3, 
     `articles`.`author` AS t0_r4, `articles`.`published` AS t0_r5, 
     `articles`.`category_id` AS t0_r6, `articles`.`issue_id` AS t0_r7, 
     `articles`.`date` AS t0_r8, `articles`.`created_at` AS t0_r9, 
     `articles`.`updated_at` AS t0_r10, `articles`.`photo_file_name` AS t0_r11, 
     `articles`.`photo_content_type` AS t0_r12, `articles`.`photo_file_size` AS t0_r13, 
     `articles`.`photo_updated_at` AS t0_r14, `categories`.`id` AS t1_r0, 
     `categories`.`name` AS t1_r1, `categories`.`permalink` AS t1_r2, 
     `categories`.`created_at` AS t1_r3, `categories`.`updated_at` AS t1_r4, 
     `categories`.`issued` AS t1_r5, `categories`.`order_articles` AS t1_r6 
     FROM `articles` LEFT OUTER JOIN `categories` ON 
     `categories`.`id` = `articles`.`category_id` WHERE 
     `articles`.`issue_id` = 409 AND `categories`.`permalink` = 'Διεθνή' LIMIT 1

在此查询中，我看到的explation使用错误的索引

+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+ 
| id | select_type | table  | type | possible_keys                | key       | key_len | ref | rows | filtered | Extra  | 
+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+ 
| 1 | SIMPLE  | categories | const | PRIMARY,index_categories_on_permalink          | index_categories_on_permalink | 768  | const | 1 | 100.00 |    | 
| 1 | SIMPLE  | articles | ref | index_articles_on_issue_id_and_category_id, index_articles_on_category_id | index_articles_on_category_id | 2  | const | 10 | 100.05 | Using where | 
+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+

我有两个索引，category_id单独和issue_id - category_id。

在此查询中，我正在使用issue_id和category_id进行搜索，这比使用index_articles_on_issue_id_and_category_id时要快得多，比index_articles_on_category_id快得多。

如何使用活动记录查询选择正确的索引？

来源

2012-12-16 mike zaby

麦克的具体的例子，你可以只用一个综合指数，以达到同样的效果。如果您只有使用category_id的查询以及其他使用category_id和issue_id（组合）的查询，那么正确的索引应该是'index_articles_on_category_id_and_issue_id'。有了这样的复合索引，你可以利用这两种查询类型。看看这个http://stackoverflow.com/questions/1823685/when-should-i-use-a-composite-index – alup

您可以方便AREL像这样使用一个索引：

class Issue 
    def self.use_index(index) 
    # update: OP fixed my mistake 
    from("#{self.table_name} USE INDEX(#{index})") 
    end 
end 

# then 
Issue.use_index("bla").where(some_condition: true)

来源

2012-12-16 18:24:48 krichard

谢谢，它适用于litle修复'from（“＃{self.table_name} USE INDEX（＃{}指数）“）' –

使用MySQL特定指数与轨道

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