我有這樣的ActiveRecord查詢使用MySQL特定指數與軌道
issue = Issue.find(id)
issue.articles.includes(:category).merge(Category.where(permalink: perma))
而轉換爲MySQL查詢
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
在此查詢中,我看到的explation使用錯誤的索引
+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+
| 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 |
+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+
我有兩個索引,category_id
單獨和issue_id - category_id
。
在此查詢中,我正在使用issue_id
和category_id
進行搜索,這比使用index_articles_on_issue_id_and_category_id
時要快得多,比index_articles_on_category_id
快得多。
如何使用活動記錄查詢選擇正確的索引?
麥克的具體的例子,你可以只用一個綜合指數,以達到同樣的效果。如果您只有使用category_id的查詢以及其他使用category_id和issue_id(組合)的查詢,那麼正確的索引應該是'index_articles_on_category_id_and_issue_id'。有了這樣的複合索引,你可以利用這兩種查詢類型。看看這個http://stackoverflow.com/questions/1823685/when-should-i-use-a-composite-index – alup