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Support ORDER BY within aggregate function calls, at long last providing a

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non-kluge method for controlling the order in which values are fed to an
aggregate function.  At the same time eliminate the old implementation
restriction that DISTINCT was only supported for single-argument aggregates.

Possibly release-notable behavioral change: formerly, agg(DISTINCT x)
dropped null values of x unconditionally.  Now, it does so only if the
agg transition function is strict; otherwise nulls are treated as DISTINCT
normally would, ie, you get one copy.

Andrew Gierth, reviewed by Hitoshi Harada
This commit is contained in:
Tom Lane
2009-12-15 17:57:48 +00:00
parent 6a6efb9640
commit 34d26872ed
31 changed files with 1184 additions and 248 deletions
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41
doc/src/sgml/func.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.492 2009/11/24 19:21:15 petere Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.493 2009/12/15 17:57:46 tgl Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
@ -8440,7 +8440,8 @@ SELECT xmlroot(xmlparse(document '<?xml version="1.1"?><content>abc</content>'),
The function <function>xmlagg</function> is, unlike the other
functions described here, an aggregate function. It concatenates the
input values to the aggregate function call,
like <function>xmlconcat</function> does.
much like <function>xmlconcat</function> does, except that concatenation
occurs across rows rather than across expressions in a single row.
See <xref linkend="functions-aggregate"> for additional information
about aggregate functions.
</para>
@ -8459,8 +8460,22 @@ SELECT xmlagg(x) FROM test;
</para>
<para>
To determine the order of the concatenation, something like the
following approach can be used:
To determine the order of the concatenation, an <literal>ORDER BY</>
clause may be added to the aggregate call as described in
<xref linkend="syntax-aggregates">. For example:
<screen><![CDATA[
SELECT xmlagg(x ORDER BY y DESC) FROM test;
xmlagg
----------------------
<bar/><foo>abc</foo>
]]></screen>
</para>
<para>
The following non-standard approach used to be recommended
in previous versions, and may still be useful in specific
cases:
<screen><![CDATA[
SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
@ -8468,9 +8483,6 @@ SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
----------------------
<bar/><foo>abc</foo>
]]></screen>
Again, see <xref linkend="functions-aggregate"> for additional
information.
</para>
</sect3>
@ -9887,20 +9899,19 @@ SELECT count(*) FROM sometable;
The aggregate functions <function>array_agg</function>
and <function>xmlagg</function>, as well as similar user-defined
aggregate functions, produce meaningfully different result values
depending on the order of the input values. In the current
implementation, the order of the input is in principle unspecified.
Supplying the input values from a sorted subquery
will usually work, however. For example:
depending on the order of the input values. This ordering is
unspecified by default, but can be controlled by writing an
<literal>ORDER BY</> clause within the aggregate call, as shown in
<xref linkend="syntax-aggregates">.
Alternatively, supplying the input values from a sorted subquery
will usually work. For example:
<screen><![CDATA[
SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
]]></screen>
But this syntax is not allowed in the SQL standard, and is
not portable to other database systems. A future version of
<productname>PostgreSQL</> might provide an additional feature to control
the order in a better-defined way (<literal>xmlagg(expr ORDER BY expr, expr,
...)</literal>).
not portable to other database systems.
</para>
<para>