Unique full text index?
Since there is a limit on the length of the index, if you want to guarantee the uniqueness of the whole field's value, you may want to add a hash field of that varchar field.

Steps:


Alter the table to add an MD5 field: alter table table1 add md5_of_column char(32)
Update this field: UPDATE table1 SET md5_of_column=md5(column);
Add the unique index: ALTER TABLE table1 ADD UNIQUE (md5_of_column)
Either create a trigger, or do it at application level, to update this new field after each insert/update operation.


Note that this solution might have a light performance hit.