<para>

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

expression that does not itself contain window function calls.

<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. <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>

</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.

The <literal>PARTITION BY</> option groups the rows of the query into

<firstterm>partitions</>, which are processed separately by the window

function. <literal>PARTITION BY</> works similarly to a query-level

<literal>GROUP BY</> clause, except that its expressions are always just

expressions and cannot be output-column names or numbers.

Without <literal>PARTITION BY</>, all rows produced by the query are

treated as a single partition.

The <literal>ORDER BY</> option determines the order in which the rows

of a partition are processed by the window function. It works similarly

to a query-level <literal>ORDER BY</> clause, but likewise cannot use

output-column names or numbers. Without <literal>ORDER BY</>, rows are

processed in an unspecified order.

</para>

<para>

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

the set of rows constituting the <firstterm>window frame</>, which is a

subset of the current partition, for those window functions that act on

the frame instead of the whole partition. The frame can be specified in

either <literal>RANGE</> or <literal>ROWS</> mode; in either case, it

runs from the <replaceable>frame_start</> to the

<replaceable>frame_end</>. If <replaceable>frame_end</> is omitted,

it defaults to <literal>CURRENT ROW</>.

</para>

<para>

A <replaceable>frame_start</> of <literal>UNBOUNDED PRECEDING</> means

that the frame starts with the first row of the partition, and similarly

a <replaceable>frame_end</> of <literal>UNBOUNDED FOLLOWING</> means

that the frame ends with the last row of the partition.

</para>

<para>

In <literal>RANGE</> mode, a <replaceable>frame_start</> of

<literal>CURRENT ROW</> means the frame starts with the current row's

first <firstterm>peer</> row (a row that <literal>ORDER BY</> considers

equivalent to the current row), while a <replaceable>frame_end</> of

<literal>CURRENT ROW</> means the frame ends with the last equivalent

peer. In <literal>ROWS</> mode, <literal>CURRENT ROW</> simply means

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

</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.

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

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

CURRENT ROW</>. With <literal>ORDER BY</>, this sets the frame to

be all rows from partition start up through the current row's

last peer. Without <literal>ORDER BY</>, all partition rows are

included in the window frame.

CURRENT ROW</>. With <literal>ORDER BY</>, this sets the frame to be

all rows from the partition start up through the current row's last

peer. Without <literal>ORDER BY</>, all rows of the partition are

included in the window frame, since all rows become peers of the current

row.

</para>

<para>