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Add some basic support for window frame clauses to the window-functions

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patch.  This includes the ability to force the frame to cover the whole
partition, and the ability to make the frame end exactly on the current row
rather than its last ORDER BY peer.  Supporting any more of the full SQL
frame-clause syntax will require nontrivial hacking on the window aggregate
code, so it'll have to wait for 8.5 or beyond.
This commit is contained in:
Tom Lane
2008-12-31 00:08:39 +00:00
parent 0fb9be7acf
commit 8e8854daa2
24 changed files with 757 additions and 307 deletions
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30
doc/src/sgml/func.sgml
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@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.464 2008/12/28 18:53:53 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.465 2008/12/31 00:08:33 tgl Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
@@ -10160,9 +10160,8 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
<para>
<firstterm>Window functions</firstterm> provide the ability to perform
calculations across sets of rows that are related to the current query
row. For information about this feature see
<xref linkend="tutorial-window"> and
<xref linkend="syntax-window-functions">.
row. See <xref linkend="tutorial-window"> for an introduction to this
feature.
</para>
<para>
@@ -10392,18 +10391,23 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
<para>
Note that <function>first_value</>, <function>last_value</>, and
<function>nth_value</> consider only the rows within the <quote>window
frame</>, that is the rows from the start of the partition through the
last peer of the current row. This is particularly likely to give
unintuitive results for <function>last_value</>.
frame</>, which by default contains the rows from the start of the
partition through the last peer of the current row. This is
likely to give unhelpful results for <function>nth_value</> and
particularly <function>last_value</>. You can redefine the frame as
being the whole partition by adding <literal>ROWS BETWEEN UNBOUNDED
PRECEDING AND UNBOUNDED FOLLOWING</> to the <literal>OVER</> clause.
See <xref linkend="syntax-window-functions"> for more information.
</para>
<para>
When an aggregate function is used as a window function, it aggregates
over the rows within the current row's window frame. To obtain
aggregation over the whole partition, be sure to omit <literal>ORDER BY</>
from the window definition. An aggregate used with <literal>ORDER BY</>
produces a <quote>running sum</> type of behavior, which may or may not
be what's wanted.
aggregation over the whole partition, omit <literal>ORDER BY</> or use
<literal>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</>.
An aggregate used with <literal>ORDER BY</> and the default window frame
definition produces a <quote>running sum</> type of behavior, which may or
may not be what's wanted.
</para>
<note>
@@ -10416,7 +10420,9 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
same as the standard's default, namely <literal>RESPECT NULLS</>.
Likewise, the standard's <literal>FROM FIRST</> or <literal>FROM LAST</>
option for <function>nth_value</> is not implemented: only the
default <literal>FROM FIRST</> behavior is supported.
default <literal>FROM FIRST</> behavior is supported. (You can achieve
the result of <literal>FROM LAST</> by reversing the <literal>ORDER BY</>
ordering.)
</para>
</note>