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)

尝试确定删除键 category_type_2 和 category_type_3 是否安全。看起来它们是与 category_type 完全相同的索引。这是一张遗留表，我不知道很久以前是谁创建的。有人最终会创建这三个看似重复的 key 是否有正当理由？

最佳答案

拥有两个相同 索引会浪费磁盘空间并且会减慢INSERTs(一点点)。没有任何好处。

您无法真正看出它们是否与 DESCRIBE TABLE 重复。相反，执行 SHOW CREATE TABLE。观察UNIQUE/not，prefixed/not，regular/FULLTEXT，手动创建/由FOREIGN KEY等创建

一旦您确定它们相同(除了名字)，就放弃一个。在其他情况下，也可能会删除索引。假设您有这 3 个索引:

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

或者这对:

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

(还有更多的组合。)

关于mysql - 在 MYSQL 中，当存在重复索引且除 key_name 之外的所有内容都相同时，这意味着什么？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/32294032/