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 name ON table (column [, ...]);
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.
