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Copy-editing for recent window-functions documentation rewrite.
Fix grammar, put back some removed information, rearrange for clarity.
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@ -1742,13 +1742,6 @@ UNBOUNDED FOLLOWING
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<para>
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Here, <replaceable>expression</replaceable> represents any value
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expression that does not itself contain window function calls.
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<literal>PARTITION BY</> behaves like a <literal>GROUP
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BY</> clause by grouping rows into partitions, except its
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expressions are always just expressions and cannot be output-column
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names or numbers. <literal>ORDER BY</> behaves similar to a
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query-level <literal>ORDER BY</> clause, except it controls the
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order rows are supplied to the aggregate function within the window
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frame, and with the same expression restrictions.
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</para>
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<para>
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@ -1766,18 +1759,49 @@ UNBOUNDED FOLLOWING
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</para>
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<para>
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In <literal>RANGE</> mode, <literal>CURRENT ROW</> starts with
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the current row's first peer that <literal>ORDER BY</>
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considers equivalent, and ends with its last equivalent peer. In
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<literal>ROWS</> mode, <literal>CURRENT ROW</> simply starts and ends
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with the current row.
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The <literal>PARTITION BY</> option groups the rows of the query into
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<firstterm>partitions</>, which are processed separately by the window
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function. <literal>PARTITION BY</> works similarly to a query-level
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<literal>GROUP BY</> clause, except that its expressions are always just
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expressions and cannot be output-column names or numbers.
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Without <literal>PARTITION BY</>, all rows produced by the query are
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treated as a single partition.
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The <literal>ORDER BY</> option determines the order in which the rows
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of a partition are processed by the window function. It works similarly
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to a query-level <literal>ORDER BY</> clause, but likewise cannot use
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output-column names or numbers. Without <literal>ORDER BY</>, rows are
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processed in an unspecified order.
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</para>
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<para>
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The <replaceable class="parameter">frame_clause</replaceable> specifies
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the set of rows constituting the <firstterm>window frame</>, which is a
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subset of the current partition, for those window functions that act on
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the frame instead of the whole partition. The frame can be specified in
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either <literal>RANGE</> or <literal>ROWS</> mode; in either case, it
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runs from the <replaceable>frame_start</> to the
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<replaceable>frame_end</>. If <replaceable>frame_end</> is omitted,
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it defaults to <literal>CURRENT ROW</>.
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</para>
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<para>
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A <replaceable>frame_start</> of <literal>UNBOUNDED PRECEDING</> means
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that the frame starts with the first row of the partition, and similarly
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a <replaceable>frame_end</> of <literal>UNBOUNDED FOLLOWING</> means
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that the frame ends with the last row of the partition.
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</para>
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<para>
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In <literal>RANGE</> mode, a <replaceable>frame_start</> of
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<literal>CURRENT ROW</> means the frame starts with the current row's
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first <firstterm>peer</> row (a row that <literal>ORDER BY</> considers
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equivalent to the current row), while a <replaceable>frame_end</> of
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<literal>CURRENT ROW</> means the frame ends with the last equivalent
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peer. In <literal>ROWS</> mode, <literal>CURRENT ROW</> simply means
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the current row.
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</para>
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<para>
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<literal>UNBOUNDED PRECEDING</> means that the frame
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starts with the first row of the partition, and similarly
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<literal>UNBOUNDED FOLLOWING</> means that the frame ends with the last
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row of the partition.
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The <replaceable>value</> <literal>PRECEDING</> and
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<replaceable>value</> <literal>FOLLOWING</> cases are currently only
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allowed in <literal>ROWS</> mode. They indicate that the frame starts
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@ -1789,15 +1813,13 @@ UNBOUNDED FOLLOWING
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</para>
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<para>
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The <replaceable class="parameter">frame_clause</replaceable> specifies
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the set of rows constituting the <firstterm>window frame</>, for those
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window functions that act on the frame instead of the whole partition.
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The default framing option is <literal>RANGE UNBOUNDED PRECEDING</>,
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which is the same as <literal>RANGE BETWEEN UNBOUNDED PRECEDING AND
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CURRENT ROW</>. With <literal>ORDER BY</>, this sets the frame to
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be all rows from partition start up through the current row's
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last peer. Without <literal>ORDER BY</>, all partition rows are
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included in the window frame.
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CURRENT ROW</>. With <literal>ORDER BY</>, this sets the frame to be
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all rows from the partition start up through the current row's last
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peer. Without <literal>ORDER BY</>, all rows of the partition are
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included in the window frame, since all rows become peers of the current
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row.
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</para>
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<para>
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