<varlistentry>

<term><literal>UNIQUE</literal> (column constraint)</term>

<term><literal>UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>

<optional> INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...]) </optional> (table constraint)</term>

<optional><literal>INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...])</literal> </optional> (table constraint)</term>

<listitem>

<para>

The <literal>UNIQUE</literal> constraint specifies that a

group of one or more columns of a table can contain

only unique values. The behavior of the unique table constraint

is the same as that for column constraints, with the additional

capability to span multiple columns.

only unique values. The behavior of a unique table constraint

is the same as that of a unique column constraint, with the

additional capability to span multiple columns. The constraint

therefore enforces that any two rows must differ in at least one

of these columns.

</para>

<para>

</para>

<para>

Each uniquetableconstraintmust name a set of columns that is

Each unique constraintshould name a set of columns that is

different from the set of columns named by any other unique or

primary key constraint defined for the table. (Otherwise it

would justbethe same constraint listed twice.)

primary key constraint defined for the table. (Otherwise, redundant

unique constraints willbediscarded.)

</para>

<para>

<para>

Adding a unique constraint will automatically create a unique btree

index on the column or group of columns used in the constraint.

The optional clause <literal>INCLUDE</literal> adds to that index

one or more columns on which the uniqueness is not enforced.

Note that although the constraint is not enforced on the included columns,

it still depends on them. Consequently, some operations on these columns

</para>

<para>

The optional <literal>INCLUDE</literal> clause adds to that index

one or more columns that are simply <quote>payload</quote>: uniqueness

is not enforced on them, and the index cannot be searched on the basis

of those columns. However they can be retrieved by an index-only scan.

Note that although the constraint is not enforced on included columns,

it still depends on them. Consequently, some operations on such columns

(e.g., <literal>DROP COLUMN</literal>) can cause cascaded constraint and

index deletion.

</para>

<varlistentry>

<term><literal>PRIMARY KEY</literal> (column constraint)</term>

<term><literal>PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] )</literal>

<optional> INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...]) </optional> (table constraint)</term>

<optional><literal>INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ...])</literal> </optional> (table constraint)</term>

<listitem>

<para>

The <literal>PRIMARY KEY</literal> constraint specifies that a column or

<para>

<literal>PRIMARY KEY</literal> enforces the same data constraints as

a combination of <literal>UNIQUE</literal> and <literal>NOT NULL</literal>, but

a combination of <literal>UNIQUE</literal> and <literal>NOT

NULL</literal>. However,

identifying a set of columns as the primary key also provides metadata

about the design of the schema, since a primary key implies that other

tables can rely on this set of columns as a unique identifier for rows.

</para>

<para>

<literal>PRIMARY KEY</literal> constraints share the restrictions that

<literal>UNIQUE</literal>constraintshave when placed on partitioned

tables.

When placed on a partitioned table,<literal>PRIMARY KEY</literal>

constraintsshare the restrictions previously decribed

for <literal>UNIQUE</literal> constraints.

</para>

<para>

Adding a <literal>PRIMARY KEY</literal> constraint will automatically

create a unique btree index on the column or group of columns used in the

constraint. The optional <literal>INCLUDE</literal> clause allows a list

of columns to be specified which will be included in the non-key portion

of the index. Although uniqueness is not enforced on the included columns,

the constraint still depends on them. Consequently, some operations on the

included columns (e.g., <literal>DROP COLUMN</literal>) can cause cascaded

constraint and index deletion.

constraint.

</para>

<para>

The optional <literal>INCLUDE</literal> clause adds to that index

one or more columns that are simply <quote>payload</quote>: uniqueness

is not enforced on them, and the index cannot be searched on the basis

of those columns. However they can be retrieved by an index-only scan.

Note that although the constraint is not enforced on included columns,

it still depends on them. Consequently, some operations on such columns

(e.g., <literal>DROP COLUMN</literal>) can cause cascaded constraint and

index deletion.

</para>

</listitem>

</varlistentry>