使用特定的mysql索引和rails

Question

我有这个ActiveRecord查询

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

并将其翻译成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

在这个查询的解释中，我看到使用了错误的索引

+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+
| 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，我使用issue_id和category_id进行搜索，使用index_articles_on_issue_id_and_category_id时比index_articles_on_category_id要快得多。

如何使用有效记录查询选择正确的索引？

Answer 1

你可以像这样使用索引来促进：

 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)

Answer 2

将use_index添加到ActiveRecord::Relation。

将some discussion添加到Rails核心已有多年，但是，看起来PR和分支都被放弃了。

如果您知道所使用的数据库及其局限性，则可以轻松地将其添加到自己的代码库中以便使用。

与@krichard的解决方案非常相似，不同之处在于它更通用化了所有使用的模型，而不仅仅是Issue。

config/initializers/active_record_relation.rb

class ActiveRecord::Relation
  # Allow passing index hints to MySQL in case the query planner gets confused.
  #
  # Example:
  #   Message.first.events.use_index( :index_events_on_eventable_type_and_eventable_id )
  #   #=> Event Load (0.5ms)  SELECT `events`.* FROM `events` USE INDEX (index_events_on_eventable_type_and_eventable_id) WHERE `events`.`eventable_id` = 123 AND `events`.`eventable_type` = 'Message'
  #
  # MySQL documentation:
  #    https://dev.mysql.com/doc/refman/5.7/en/index-hints.html
  #
  # See https://github.com/rails/rails/pull/30514
  #
  def use_index( index_name )
    self.from( "#{ self.quoted_table_name } USE INDEX ( #{ index_name } )" )
  end
end

这将允许您使用类似的内容：

issue.articles.includes( :category ).use_index( :index_articles_on_issue_id_and_category_id )

生成的SQL将包括：

FROM articles USE INDEX( index_articles_on_issue_id_and_category_id )