nat*_*odd 6 mysql sql indexing
有一个典型的Users表，其中包含字段：id (primary)、application_id、login、phone等（application_id- 选择性字段）
有几个索引：
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`.*
FROM `users`
WHERE `users`.`application_id` = 1234
LIMIT 10 OFFSET 0;
棘手的部分是该查询使用两个唯一索引之一（unique_index_users_on_application_id_and_login例如），然后返回按 排序的用户列表login。但我需要将它们按 排序id。
为此，我更新了查询：
SELECT `users`.*
FROM `users`
WHERE `users`.`application_id` = 1234
ORDER BY id
LIMIT 10 OFFSET 0;
好吧，现在解释显示 MySQL 开始使用PRIMARY键而不是任何索引。但这是怎么发生的呢？如果index_users_on_application_id实际上应该包含两个字段：[application_id, id] (InnoDB)，那么该索引非常适合查询，但 MySQL 决定选择另一个字段。
如果我说IGNORE INDEX(PRIMARY)，MySQL开始使用unique_index_users_on_application_id_and_login，仍然忽略正确的索引。时的结果相同ORDER BY id+0。
我还尝试ORDER BY application_id, id确保索引最适合，MySQL 仍然选择错误的索引。
有什么想法，为什么会发生以及如何确保 MySQL 使用正确的索引而无需明确说明USE INDEX(index_users_on_application_id)？
表索引的完整列表Users：
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)
解释示例：
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)
问题本身在于，使用错误的索引会导致类似的查询（限制为 100 而不是 1）执行几分钟，而使用正确的索引则只需一瞬间的事情。
分析：
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用法：
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] ...
对于您的情况，我认为最好的解决方案是按名称命名属性，而不是使用星号并直接在查询中*使用：IGNORE INDEX (unique_index_users_on_application_id_and_login, unique_index_users_on_application_id_and_phone ) FOR ORDER BY
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
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