UNIQUE constraints specify that the column set they modify must contain unique values. Teradata Database implements UNIQUE constraints as either a unique secondary indexes or as a single‑table join index.
The following rules apply to UNIQUE constraints.
You cannot define a UNIQUE constraint for an unpartitioned NoPI table.
Otherwise, Teradata Database does not return an error, but instead converts the UNIQUE constraint specification to a UNIQUE NOT NULL secondary index specification. As a result of this conversion, if you submit a SHOW TABLE request on such a table, the create text that the system returns does not show a UNIQUE constraint specification, but instead returns a UNIQUE NOT NULL secondary index or single‑table join index specification on the column set that had been specified for the UNIQUE constraint.
Note that this is different from the ordinary implementation of a UNIQUE constraint column set as a UNIQUE NOT NULL secondary index because Teradata Database actually changes the stored create text for a NoPI table defined with a UNIQUE constraint, it does not simply implement the constraint as a USI.