Indexes may also be used to enforce uniqueness of a column's value, or the uniqueness of the combined values of more than one column.
CREATE UNIQUE INDEX
Currently, only B-tree indexes can be declared unique.
When an index is declared unique, multiple table rows with equal indexed values will not be allowed. Null values are not considered equal. A multicolumn unique index will only reject cases where all of the indexed columns are equal in two rows.
PostgreSQL automatically creates a unique index when a unique constraint or a primary key is defined for a table. The index covers the columns that make up the primary key or unique columns (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.
Note: The preferred way to add a unique constraint to a table is ALTER TABLE ... ADD CONSTRAINT. The use of indexes to enforce unique constraints could be considered an implementation detail that should not be accessed directly. One should, however, be aware that there's no need to manually create indexes on unique columns; doing so would just duplicate the automatically-created index.