There is a new requirement in v15 for unique fields. In v15, unique fields must be indexed. If the field has the unique property checked, an index is automatically created. To remove the index, the unique attribute should be removed first. In the image below, the inspector is shown for a unique field. Note that in the indexing drop down, the option for "none" does not exist.



In previous versions of 4D, it was possible to have unique fields with no indexes. If the structure is then upgraded to v15, a B-tree index will be created for unique fields that previously had no indexes.