在MYSQL中,当存在重复索引时,除了key_name之外的所有内容都相同时,它意味着什么？
作者：互联网
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)
试图找出删除密钥category_type_2和category_type_3是否安全.看起来它们与category_type完全相同.这是一张遗留表,我不知道很久以前是谁创建的.是否有任何正当理由有人最终会创建这三个似乎重复的密钥？
解决方法:
具有两个相同的索引是浪费磁盘空间并减慢INSERT(一点点).没有任何好处.
您无法真正看到它们是否与DESCRIBE TABLE重复.相反,请执行SHOW CREATE TABLE.注意UNIQUE / not,prefixed / not,regular / FULLTEXT,手动创建/由FOREIGN KEY创建,等等.
一旦您确定它们是相同的(名称除外),请删除一个.在其他情况下,也可以删除索引.假设你有这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,考虑这一对：
INDEX(a,b) -- keep; provides composite index
UNIQUE(a) -- keep; provides uniqueness check
(还有更多组合.)
标签：unique-index,mysql,indexing,database-administration 来源： https://codeday.me/bug/20190727/1557787.html