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Diffstat (limited to 'doc/src/sgml/func.sgml')
| -rw-r--r-- | doc/src/sgml/func.sgml | 22 |
1 files changed, 12 insertions, 10 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index fed003c4d01..a4ea1462a8b 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 1.501 2010/02/07 20:48:09 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.502 2010/02/12 17:33:19 tgl Exp $ --> <chapter id="functions"> <title>Functions and Operators</title> @@ -10559,21 +10559,23 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab; <function>nth_value</> consider only the rows within the <quote>window 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. + likely to give unhelpful results for <function>last_value</> and + sometimes also <function>nth_value</>. You can redefine the frame by + adding a suitable frame specification (<literal>RANGE</> or + <literal>ROWS</>) to the <literal>OVER</> clause. + See <xref linkend="syntax-window-functions"> for more information + about frame specifications. </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, omit <literal>ORDER BY</> or use - <literal>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</>. + over the rows within the current row's window frame. 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. + may not be what's wanted. To obtain + aggregation over the whole partition, omit <literal>ORDER BY</> or use + <literal>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</>. + Other frame specifications can be used to obtain other effects. </para> <note> |