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.
ALTER TABLE ADD CONSTRAINT may be the preferred way to add a constraint, but to add a partial constraint, you can only use a partial index.
Example : ton ensure the uniqueness of a list of columns, where one of these columns is > to 0.
If you want to create a unique index on a table that already contains duplicate values you can find them using a query like the following one:
SELECT username, subscriber FROM users GROUP BY username, subscriber HAVING count(*) > 1;
and then delete or update those entries.