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.
PostgreSQL automatically creates unique
indexes when a table is declared with a unique constraint or a
primary key, on the columns that make up the primary key or unique
columns (a multicolumn index, if appropriate), to enforce that
constraint. A unique index can be added to a table at any later
time, to add a unique 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.
