A data.table's key need not be unique, but there are frequently circumstances
where non-unique keys can wreak havoc.
has_unique_key reports the existence of a unique key, and
set_unique_key both sets and ensures the uniqueness of keys.
has_unique_key(DT) set_unique_key(DT, ...)
| DT | A data.table |
|---|---|
| ... | keys to set |
has_unique_key returns TRUE if DT has a unique key, FALSE otherwise.
set_unique_key runs setkey(DT, ...) then checks whether the key is unique, returning the keyed
data.table if the key is unique, or an error message otherwise.