Indexes can 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 [, ...])
[INCLUDING (column [, ...])];

Currently, only B-tree indexes can be declared unique.

When an index is declared unique, multiple table rows with equal indexed values are not allowed. Null values are not considered equal. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows. Columns included with clause INCLUDING aren't used to enforce constraints (UNIQUE, PRIMARY KEY, etc).

Postgres Pro automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.