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.
