在MYSQL中，当重复的索引中除key_name之外的所有内容都相同时，这是什么意思？

Question

describe etc_category_metadata;

+---------------------+---------------+------+-----+---------+----------------+
| Field               | Type          | Null | Key | Default | Extra          |
+---------------------+---------------+------+-----+---------+----------------+
| id                  | int(11)       | NO   | PRI | NULL    | auto_increment |
| user_id             | bigint(20)    | NO   |     | NULL    |                |
| time_updated        | int(11)       | YES  |     | NULL    |                |
| category_type       | int(11)       | YES  | MUL | NULL    |                |
| status_keywords     | mediumblob    | YES  |     | NULL    |                |
| page_keywords       | mediumblob    | YES  |     | NULL    |                |
| profession_keywords | mediumblob    | YES  |     | NULL    |                |
| adgroup_ids         | mediumblob    | YES  |     | NULL    |                |
| prod                | tinyint(1)    | YES  |     | 0       |                |
| version             | int(11)       | YES  |     | 1       |                |
| status              | int(11)       | YES  |     | 0       |                |
| dep_category_ids    | mediumblob    | YES  |     | NULL    |                |
| custom_param        | mediumblob    | YES  |     | NULL    |                |
| queue_priority      | int(11)       | YES  |     | 1       |                |
| auto_requeue_num    | int(11)       | YES  |     | 0       |                |
| cloned_version      | int(11)       | YES  |     | 0       |                |
| custom_query        | varchar(1000) | YES  |     | NULL    |                |
| description         | varchar(1000) | YES  |     | NULL    |                |
| error_message       | mediumblob    | YES  |     | NULL    |                |
| time_last_completed | int(11)       | YES  |     | NULL    |                |
+---------------------+---------------+------+-----+---------+----------------+
21 rows in set (0.40 sec)

show index from etc_category_metadata;
+-----------------------+------------+-----------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table                 | Non_unique | Key_name        | Seq_in_index | Column_name   | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------------+------------+-----------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| etc_category_metadata |          0 | PRIMARY         |            1 | id            | A         |       12613 |     NULL | NULL   |      | BTREE      |         |               |
| etc_category_metadata |          0 | category_type   |            1 | category_type | A         |       12613 |     NULL | NULL   | YES  | BTREE      |         |               |
| etc_category_metadata |          0 | category_type   |            2 | version       | A         |       12613 |     NULL | NULL   | YES  | BTREE      |         |               |
| etc_category_metadata |          0 | category_type_2 |            1 | category_type | A         |       12613 |     NULL | NULL   | YES  | BTREE      |         |               |
| etc_category_metadata |          0 | category_type_2 |            2 | version       | A         |       12613 |     NULL | NULL   | YES  | BTREE      |         |               |
| etc_category_metadata |          0 | category_type_3 |            1 | category_type | A         |       12613 |     NULL | NULL   | YES  | BTREE      |         |               |
| etc_category_metadata |          0 | category_type_3 |            2 | version       | A         |       12613 |     NULL | NULL   | YES  | BTREE      |         |               |
+-----------------------+------------+-----------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
7 rows in set (0.07 sec)

试图找出是否是安全的删除键category_type_2和category_type_3。看来它们是与完全相同的索引category_type。这是一个旧表，我不知道是谁在很久以前创建的。有人最终会创建似乎重复的这三个键是否有任何正当理由？

Answer 1

具有两个相同的索引会浪费磁盘空间，并且会减慢速度INSERTs（有一点）。没有好处。

您真的看不到它们是否与重复DESCRIBE TABLE。相反，请执行SHOW CREATE TABLE。注意UNIQUE/ not，前缀/ not，regular / FULLTEXT，手动创建/由FOREIGN KEY等等创建的。

一旦确定它们相同（名称除外），就放一。在其他情况下，索引也可能会被删除。假设您具有以下三个索引：

INDEX(a,b)  -- keep this
INDEX(a)    -- unnecessary
INDEX(b)    -- keep

或这对：

UNIQUE(a)   -- keep; same as INDEX(a), plus a uniqueness check
INDEX(a)    -- drop

更微妙的是，考虑这对：

INDEX(a,b)  -- keep; provides composite index
UNIQUE(a)   -- keep; provides uniqueness check

（还有更多组合。）