Unique composite key on rows

[cgrand]

Given a query how do I determine a subset of columns that are enough to uniquely identify a row?

[cgrand]

For example:

[:ul
 (with {:db/id item [title done] :item/attrs}
   [:li title])]

This query returns 3 columns but item is enough for the identity of the row.

[cgrand]

All columns which are cardinality-one attributes of another column are not key. It works transitively too.