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Rewrite window function reference section to more clearly explain

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keywords and concepts, based on suggestions by Florian Pflug.
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Bruce Momjian 2012-08-16 10:42:42 -04:00
parent 7c97f6a0cb
commit d5816d11af
1 changed files with 41 additions and 27 deletions
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doc/src/sgml/syntax.sgml
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@ -1742,11 +1742,16 @@ UNBOUNDED FOLLOWING
<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
@ -1760,39 +1765,48 @@ UNBOUNDED FOLLOWING
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>