MySQL使用了不正确的索引。为什么？
有一个典型的MySQL使用了不正确的索引。为什么？,mysql,sql,indexing,Mysql,Sql,Indexing,有一个典型的Users表，其中包含以下字段：id（primary），application\u id，login，phone，等等（application\u id-选择性字段） 索引很少： index_users_on_application_id, unique_index_users_on_application_id_and_login unique_index_users_on_application_id_and_phone 查询本身非常简单： SELECT `users`.*
Usersid（primary）application\u idloginphoneapplication\u idindex_users_on_application_id,
unique_index_users_on_application_id_and_login
unique_index_users_on_application_id_and_phone
SELECT `users`.*
FROM `users`
WHERE `users`.`application_id` = 1234
LIMIT 10 OFFSET 0;
unique\u index\u users\u on\u application\u id\u和\u loginloginidSELECT `users`.*
FROM `users`
WHERE `users`.`application_id` = 1234
ORDER BY id
LIMIT 10 OFFSET 0;
PRIMARYindex\u users\u on\u application\u idignoreindex（PRIMARY）unique\u INDEX\u users\u on\u application\u id\u和\u login使用索引（index\u users\u on\u application\u id）用户索引的完整列表mysql> show indexes from users;
+-------+------------+-----------------------------------------------------+--------------+----------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------------------------------------------+--------------+----------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| users | 0 | PRIMARY | 1 | id | A | 21893 | NULL | NULL | | BTREE | | |
| users | 0 | index_users_on_confirmation_token | 1 | confirmation_token | A | 28 | NULL | NULL | YES | BTREE | | |
| users | 0 | index_users_on_reset_password_token | 1 | reset_password_token | A | 50 | NULL | NULL | YES | BTREE | | |
| users | 0 | index_users_on_application_id_and_external_user_id | 1 | application_id | A | 32 | NULL | NULL | YES | BTREE | | |
| users | 0 | index_users_on_application_id_and_external_user_id | 2 | external_user_id | A | 995 | NULL | NULL | YES | BTREE | | |
| users | 0 | index_users_on_application_id_and_login | 1 | application_id | A | 30 | NULL | NULL | YES | BTREE | | |
| users | 0 | index_users_on_application_id_and_login | 2 | login | A | 21893 | NULL | NULL | YES | BTREE | | |
| users | 1 | users_account_id_fk | 1 | account_id | A | 44 | NULL | NULL | | BTREE | | |
| users | 1 | users_blob_id_fk | 1 | blob_id | A | 118 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_remember_token | 1 | remember_token | A | 8 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id | 1 | application_id | A | 32 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_facebook_id | 1 | application_id | A | 32 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_facebook_id | 2 | facebook_id | A | 3127 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_twitter_digits_id | 1 | application_id | A | 32 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_twitter_digits_id | 2 | twitter_digits_id | A | 138 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_email | 1 | application_id | A | 32 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_email | 2 | email | A | 2189 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_full_name | 1 | application_id | A | 32 | NULL | NULL | YES | BTREE | | |
| users | 1 | index_users_on_application_id_and_full_name | 2 | full_name | A | 5473 | NULL | NULL | YES | BTREE | | |
+-------+------------+-----------------------------------------------------+--------------+----------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
19 rows in set (0.01 sec)
mysql> EXPLAIN SELECT `users`.* FROM `users` WHERE `users`.`application_id` = 56374 ORDER BY id asc LIMIT 1 OFFSET 0;
+----+-------------+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------+---------+-------+------+-----------------------------+
| 1 | SIMPLE | users | ref | index_users_on_application_id_and_external_user_id,index_users_on_application_id_and_login,index_users_on_application_id,index_users_on_application_id_and_facebook_id,index_users_on_application_id_and_twitter_digits_id,index_users_on_application_id_and_email,index_users_on_application_id_and_full_name | index_users_on_application_id_and_external_user_id | 5 | const | 1 | Using where; Using filesort |
+----+-------------+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------+---------+-------+------+-----------------------------+
1 row in set (0.00 sec)
SET PROFILING = 1; SELECT `users`.* FROM `users` WHERE `users`.`application_id` = 56374 ORDER BY id asc LIMIT 1 OFFSET 0; SHOW PROFILE FOR QUERY 1; SET PROFILING = 0;
Query OK, 0 rows affected (0.00 sec)
+----------+----------+-----------------+-------+-------+----------------------------------------------------------------------------------------------------------------------------------+----------------------+-------+---------+---------------------+---------------------+---------------------+------------------+-------------+------------+---------+----------------------+------------------------+---------------------+--------------------+---------------------+----------------------+-------------------+------------+----------------+-------------+---------------------------------------------------------------------------------------+-------------------+-------------------------+--------------------------------------------------+----------------+
-- fields list --
+----------+----------+-----------------+-------+-------+----------------------------------------------------------------------------------------------------------------------------------+----------------------+-------+---------+---------------------+---------------------+---------------------+------------------+-------------+------------+---------+----------------------+------------------------+---------------------+--------------------+---------------------+----------------------+-------------------+------------+----------------+-------------+---------------------------------------------------------------------------------------+-------------------+-------------------------+--------------------------------------------------+----------------+
| 27265241 | NULL | Some Username | NULL | 9777 | SomeHash | AnotherHash | NULL | NULL | 2017-04-12 15:53:32 | 2017-09-21 13:39:51 | 2017-09-24 19:19:06 | 1234 | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 2017-07-05 10:59:59 | NULL | NULL | 12345 | NULL | NULL | something_else | NULL | 1 | another_hash | 54321 |
+----------+----------+-----------------+-------+-------+----------------------------------------------------------------------------------------------------------------------------------+----------------------+-------+---------+---------------------+---------------------+---------------------+------------------+-------------+------------+---------+----------------------+------------------------+---------------------+--------------------+---------------------+----------------------+-------------------+------------+----------------+-------------+---------------------------------------------------------------------------------------+-------------------+-------------------------+--------------------------------------------------+----------------+
1 row in set (1 min 14.43 sec)
+--------------------------------+-----------+
| Status | Duration |
+--------------------------------+-----------+
| starting | 0.000068 |
| Waiting for query cache lock | 0.000025 |
| init | 0.000025 |
| checking query cache for query | 0.000047 |
| checking permissions | 0.000026 |
| Opening tables | 0.000031 |
| After opening tables | 0.000025 |
| System lock | 0.000025 |
| Table lock | 0.000026 |
| Waiting for query cache lock | 0.000037 |
| init | 0.000046 |
| optimizing | 0.000032 |
| statistics | 0.000225 |
| preparing | 0.000042 |
| executing | 0.000025 |
| Sorting result | 0.000057 |
| Sending data | 42.952100 |
| end | 0.000070 |
| query end | 0.000027 |
| closing tables | 0.000025 |
| Unlocking tables | 0.000028 |
| freeing items | 0.000028 |
| updating status | 0.000039 |
| cleaning up | 0.000025 |
+--------------------------------+-----------+
24 rows in set (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
您应该能够使用索引提示和优化器提示来建议正确的索引使用：
您可以直接提示该表：
tbl_name [[AS] alias] [index_hint_list]
index_hint_list:
index_hint [index_hint] ...
index_hint:
USE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
| IGNORE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
| FORCE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
index_list:
index_name [, index_name] ...
*忽略索引（应用程序id上的唯一索引用户和登录，应用程序id上的唯一索引用户和电话）直接在查询中进行订购：
SELECT `users`.*
FROM `users`
WHERE `users`.`application_id` = 1234
ORDER BY id
LIMIT 10 OFFSET 0;
基于您的代码的示例：
SELECT u.id,
u.application_id,
u.login,
u.phone,
# ... here to continue
FROM users as u
IGNORE INDEX (unique_index_users_on_application_id_and_login, unique_index_users_on_application_id_and_phone ) FOR ORDER BY
WHERE u.application_id = 1234
ORDER BY u.id
LIMIT 10 OFFSET 0;
第一次编辑
由于下面的评论，我添加了一个使主键无效的技巧
您还可以通过以下技巧使主键无效：
SELECT u.id,
u.application_id,
u.login,
u.phone,
#...
FROM users as u
WHERE u.application_id = 1234
ORDER BY u.id+0
很难确定，但在很多情况下，MySQL擅长选择正确的索引
可能有必要为查询分析器提供一些有用的信息—有时候查询分析器没有关于数据的好信息，这可能会导致奇怪的行为
但是…您的应用程序id上的索引用户\u仅具有应用程序id
，而不具有id
我猜application\u id
具有相当少的不同值。列ID
是唯一的，其值与表中的行数相同。没有包含where子句和ORDERBY子句的索引，因此MySQL猜测最昂贵的部分将是按ID
排序，而不是按application\u ID
筛选
所以，我认为MySQL在这里做的是正确的。在幕后，它将返回按ID排序的所有行，并遍历该列表，并为您提供具有指定应用程序ID的前10行
显然，要做的事情是创建一个带有应用程序id和id的索引，看看MySQL是否会选择它。当您使用orderbycolumn\u name
MySQL扫描ORDERBY子句中给定列中的所有数据时，在您的例子中，id
column将得到扫描。对于此数据库，请使用表指针。在InnoDB中，它是主键的值；在MyISAM中，它是.MYD文件中的偏移量
这就是为什么当您使用orderbyid时，您的查询开始只使用主键作为索引
要使用您创建的索引，请添加id
列作为索引的第一列。然后它将有效地使用索引。因此，应用程序id和用户登录表上的索引unique\u index\u users\u应按相同顺序1-id和2-application\u id包含以下列
有关ORDER BY/LIMIT的MySQL性能的更多详细信息，在这种情况下，不应该使用索引，甚至强制索引（correct_INDEX）更有效，因为您已经指定了索引？@milangelebit Yes USE&FORCE会更有效。我坚持希望索引，但没有明确说明使用索引（index\u users\u on\u application\u id）
。看到你回答了这个问题，你能告诉我实际的问题是什么，你的答案为OP解决了什么吗？@Mjh因为在我回答之前没有编辑，我不得不猜测nattfold想要对优化器使用索引（在编辑之前回答）。我的回答使他能够这样做，即使我现在看到的成本更高（以前也没有编辑）。是的，但我想问的是，你是否理解他的问题到底是什么？我不。我不明白如果他强制使用一个索引或使用另一个索引会发生什么。解决了什么问题？到目前为止，我还没有遇到过这样的情况，MySQL必须被明确告知不要使用主键而使用其他索引。很难理解你的问题-你能发布这个表的完整DDL吗，包括索引，并显示查询计划？添加了索引列表和解释example@Mjh我想这里的问题是这个查询（使用选定的主索引）需要大约30秒。但当与use INDEX（INDEX\u users\u on\u application\u id）一起使用时，需要0秒。问题是为什么？为什么MySQL会选择这个慢索引？@Mjh这是一个现实世界的问题，“正确”和“不正确”索引之间的区别就像查询执行时间的分钟数。请注意，Profile
是多么无用？它通常说最多的时间是在发送数据时。（事实证明，这不是n