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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.
</p><pre class="synopsis">
CREATE UNIQUE INDEX <em class="replaceable"><code>name</code></em> ON <em class="replaceable"><code>table</code></em> (<em class="replaceable"><code>column</code></em> [<span class="optional">, ...</span>]);
</pre><p>
Currently, only B-tree indexes can be declared unique.
</p><p>
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.
</p><p>
<span class="productname">PostgreSQL</span> 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.
</p><div class="note"><h3 class="title">Note</h3><p>
There's no need to manually
create indexes on unique columns; doing so would just duplicate
the automatically-created index.
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