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Copy-editing for recent window-functions documentation rewrite.

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Fix grammar, put back some removed information, rearrange for clarity.
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Tom Lane 2012-08-17 15:40:35 -04:00
parent 470d0b9789
commit 18226849ea
1 changed files with 45 additions and 23 deletions
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68
doc/src/sgml/syntax.sgml
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@ -1742,13 +1742,6 @@ UNBOUNDED FOLLOWING
<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>
@ -1766,18 +1759,49 @@ UNBOUNDED FOLLOWING
</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
@ -1789,15 +1813,13 @@ UNBOUNDED FOLLOWING
</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>