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Diffstat (limited to 'doc/src/sgml/func.sgml')
| -rw-r--r-- | doc/src/sgml/func.sgml | 30 |
1 files changed, 18 insertions, 12 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index 205b71e9c9..bf3293a0cf 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -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> |