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.

<synopsis>

CREATE UNIQUE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <optional>, ...</optional>);

CREATE UNIQUE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <optional>, ...</optional>) <optional> NULLS <optional> NOT </optional> DISTINCT </optional>;

</synopsis>

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

</para>

<para>

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.

indexed values are not allowed. By default, null values in a unique column

are not considered equal, allowing multiple nulls in the column. The

<literal>NULLS NOT DISTINCT</literal> 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.

</para>

<para>