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>11.6. Unique Indexes</A
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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 <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> ON <TT
CLASS="REPLACEABLE"
><I
>table</I
></TT
> (<TT
CLASS="REPLACEABLE"
><I
>column</I
></TT
> [<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
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><B
>Note: </B
> The preferred way to add a unique constraint to a table is
<TT
CLASS="LITERAL"
>ALTER TABLE ... ADD CONSTRAINT</TT
>. 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.
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