---
title: "Unique Indexes"
id: indexes-unique
pg_version: "20devel"
---

<a id="indexes-unique"></a>

## 11.6. Unique Indexes

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 , ...)  NULLS  NOT  DISTINCT ;
```

 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. By default, null values in a unique column are not considered equal, allowing multiple nulls in the column. The `NULLS NOT DISTINCT` option modifies this and causes the index to treat nulls as equal. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows. 

PostgreSQL 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. 


> [!NOTE]
> There's no need to manually create indexes on unique columns; doing so would just duplicate the automatically-created index.