c8h4/postgresql
1
0
Fork 0
Code Issues Pull requests Actions Packages Projects Releases Wiki Activity

Rewrite window function reference section to more clearly explain

Browse source 
keywords and concepts, based on suggestions by Florian Pflug.
This commit is contained in:
Bruce Momjian 2012-08-16 10:42:42 -04:00
parent 7c97f6a0cb
commit d5816d11af
1 changed files with 41 additions and 27 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

68
doc/src/sgml/syntax.sgml
View file

@ -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>