Question

I have the following table on a server:

CREATE TABLE routes (
  id          int(11) unsigned NOT NULL AUTO_INCREMENT,
  from_lat    double NOT NULL,
  from_lng    double NOT NULL,
  to_lat      double NOT NULL,
  to_lng      double NOT NULL,
  distance    int(11) unsigned NOT NULL,
  drive_time  int(11) unsigned NOT NULL,

  PRIMARY KEY (id),
  UNIQUE KEY route (from_lat,from_lng,to_lat,to_lng)
) ENGINE=InnoDB;

We are saving some routing information from point A (from_lat, from_lng) to point B (to_lat, to_lng). There is a unique index on the coordinates.

However, there are two entries in the database that confuse me:

+----+----------+----------+---------+---------+----------+------------+
| id | from_lat | from_lng | to_lat  | to_lng  | distance | drive_time |
+----+----------+----------+---------+---------+----------+------------+
| 27 | 52.5333  | 13.1667  | 52.5833 | 13.2833 | 13647    | 1125       |
| 28 | 52.5333  | 13.1667  | 52.5833 | 13.2833 | 13647    | 1125       |
+----+----------+----------+---------+---------+----------+------------+

They are exactly the same.

When I not try to export the database using mysqldump and trying to reimport it, I get an error:

ERROR 1062 (23000): Duplicate entry '52.5333-13.1667-52.5833-13.2833' for key 'route'

How can it be that this is in the database, when there is an unique key on them? Shouldn’t MySQL reject them?

Answer 1

Is it possible that the double values are slightly different, but only after the 4th digit?
If you export and import them, they would be the same, and that would give a unique constraint violation.

Quoting from this MySQL bug report:

When mysqldump dumps a DOUBLE value, it uses insufficient precision to
distinguish between some close values (and, presumably, insufficient
precision to recreate the exact values from the original database). If
the DOUBLE value is a primary key or part of a unique index, restoring
the database from this output fails with a duplicate key error.

Try to display them with more digits behind the comma (how will depend on your client.)