How indexes affect primary-key, unique, and referential constraints

The database server creates internal B-tree indexes for primary-key, unique, and referential constraints. If a primary-key, unique, or referential constraint is added after the table is created, any user-created indexes on the constrained columns are used, if appropriate.

An appropriate index is one that indexes the same columns that are used in the primary-key, referential, or unique constraint.

If an appropriate user-created index is not available, the database server creates a nonfragmented internal index on the constrained column or columns.