<para>

Here, <replaceable>expression</replaceable> represents any value

expression that does not itself contain window function calls.

The <literal>PARTITION BY</> and <literal>ORDER BY</> lists have

essentially the same syntax and semantics as <literal>GROUP BY</>

and <literal>ORDER BY</> clauses of the whole query, except that their

<literal>PARTITION BY</> behaves like a <literal>GROUP

BY</> clause by grouping rows into partitions, except its

expressions are always just expressions and cannot be output-column

names or numbers.

names or numbers. <literal>ORDER BY</> behaves similar to a

query-level <literal>ORDER BY</> clause, except it controls the

order rows are supplied to the aggregate function within the window

frame, and with the same expression restrictions.

</para>

<para>

<replaceable>window_name</replaceable> is a reference to a named window

specification defined in the query's <literal>WINDOW</literal> clause.

Named window specifications are usually referenced with just

page for details.

</para>

<para>

In <literal>RANGE</> mode, <literal>CURRENT ROW</> starts with

the current row's first peer that <literal>ORDER BY</>

considers equivalent, and ends with its last equivalent peer. In

<literal>ROWS</> mode, <literal>CURRENT ROW</> simply starts and ends

with the current row.

</para>

<para>

<literal>UNBOUNDED PRECEDING</> means that the frame

starts with the first row of the partition, and similarly

<literal>UNBOUNDED FOLLOWING</> means that the frame ends with the last

row of the partition.

The <replaceable>value</> <literal>PRECEDING</> and

<replaceable>value</> <literal>FOLLOWING</> cases are currently only

allowed in <literal>ROWS</> mode. They indicate that the frame starts

or ends the specified number of rows before or after the current row.

<replaceable>value</replaceable> must be an integer expression not

containing any variables, aggregate functions, or window functions.

The value must not be null or negative; but it can be zero, which

just selects the current row.

</para>

<para>

The <replaceable class="parameter">frame_clause</replaceable> specifies

the set of rows constituting the <firstterm>window frame</>, for those

window functions that act on the frame instead of the whole partition.

If <replaceable>frame_end</> is omitted it defaults to <literal>CURRENT

ROW</>. Restrictions are that

The default framing option is <literal>RANGE UNBOUNDED PRECEDING</>,

which is the same as <literal>RANGE BETWEEN UNBOUNDED PRECEDING AND

CURRENT ROW</>. This sets the frame to be all rows from the partition start

up through the current row's last peer as specified by <literal>ORDER BY</>,

or all partition rows if there is no <literal>ORDER BY</>.

</para>

<para>

Restrictions are that

<replaceable>frame_start</> cannot be <literal>UNBOUNDED FOLLOWING</>,

<replaceable>frame_end</> cannot be <literal>UNBOUNDED PRECEDING</>,

and the <replaceable>frame_end</> choice cannot appear earlier in the

above list than the <replaceable>frame_start</> choice &mdash; for example

<literal>RANGE BETWEEN CURRENT ROW AND <replaceable>value</>

PRECEDING</literal> is not allowed.

The default framing option is <literal>RANGE UNBOUNDED PRECEDING</>,

which is the same as <literal>RANGE BETWEEN UNBOUNDED PRECEDING AND

CURRENT ROW</>; it sets the frame to be all rows from the partition start

up through the current row's last peer in the <literal>ORDER BY</>

ordering (which means all rows if there is no <literal>ORDER BY</>).

In general, <literal>UNBOUNDED PRECEDING</> means that the frame

starts with the first row of the partition, and similarly

<literal>UNBOUNDED FOLLOWING</> means that the frame ends with the last

row of the partition (regardless of <literal>RANGE</> or <literal>ROWS</>

mode). In <literal>ROWS</> mode, <literal>CURRENT ROW</>

means that the frame starts or ends with the current row; but in

<literal>RANGE</> mode it means that the frame starts or ends with

the current row's first or last peer in the <literal>ORDER BY</> ordering.

The <replaceable>value</> <literal>PRECEDING</> and

<replaceable>value</> <literal>FOLLOWING</> cases are currently only

allowed in <literal>ROWS</> mode. They indicate that the frame starts

or ends with the row that many rows before or after the current row.

<replaceable>value</replaceable> must be an integer expression not

containing any variables, aggregate functions, or window functions.

The value must not be null or negative; but it can be zero, which

selects the current row itself.

</para>

<para>