Table with unique constraint has duplicate records (MySQL 5.1.57)

问题

I recently ran into a quite puzzling situation. In a table which has a unique constraint set on 2 fields, there are many existing records that violate this constraint!

And the main question is: How could they have gotten there in the first place, with the unique constraint already in place?

Schema:

CREATE TABLE `rest_service_mediafile` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `video_id` int(11) NOT NULL,
  `video_encoding_id` int(11) NOT NULL,
  `external_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `rest_service_mediafile_video_id_4a2efdd4_uniq` (`video_id`,`video_encoding_id`),
  KEY `rest_service_mediafile_5d9d774` (`video_id`),
  KEY `rest_service_mediafile_62de6709` (`video_encoding_id`),
  CONSTRAINT `video_encoding_id_refs_id_3122bd65` FOREIGN KEY (`video_encoding_id`) REFERENCES `rest_service_videoencoding` (`id`),
  CONSTRAINT `video_id_refs_id_567c8704` FOREIGN KEY (`video_id`) REFERENCES `rest_service_video` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=93154 DEFAULT CHARSET=utf8

Sample query: (but this is not the only duplicate)

mysql> select id, video_id, video_encoding_id, external_id from rest_service_mediafile where video_id = 157165 and video_encoding_id=2;
+-------+----------+-------------------+-------------+
| id    | video_id | video_encoding_id | external_id |
+-------+----------+-------------------+-------------+
| 82416 |   157165 |                 2 |           0 | 
| 82424 |   157165 |                 2 |           0 | 
+-------+----------+-------------------+-------------+

And if I try an alter table query on some field like:

ALTER TABLE rest_service_mediafile MODIFY external_id varchar(255) NULL;

I will get the constraint error: The error was: (1062, "Duplicate entry '157165-2' for key 'rest_service_mediafile_video_id_4a2efdd4_uniq'")

Also if I try to simply insert a duplicate value, it will fail correctly! Any of you encountered a scenario in which this is possible in MySQL?

回答1:

Was the constraint added post hoc?

Bug: ALTER IGNORE TABLE t ADD UNIQUE INDEX does not delete duplicate rows in MySQL 5.1 and 5.5.

This is noted in the InnoDB docs as a Limitation of Fast Index Creation.

来源：https://stackoverflow.com/questions/12099230/table-with-unique-constraint-has-duplicate-records-mysql-5-1-57

标签

mysql

unique-constraint

duplicate-data