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<chapter id="functions">

<title>Functions and Operators</title>

</para>

<para>

The influence the order of the concatenation, something like the

following approachto sort the input valuescan be used:

To determine the order of the concatenation, something like the

following approach can be used:

<screen><![CDATA[

SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;

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 theconcatenation is in principle

undefined. Makingthe input valuesto be sortedin some other way

implementation, the order of theinput is in principle unspecified.

Supplyingthe input valuesfrom a sortedsubquery

will usually work, however. For example:

<screen><![CDATA[

SELECT xmlagg(x) FROM (SELECT* FROM test ORDER BY y DESC) AS tab;

SELECT xmlagg(x) FROM (SELECTx FROM test ORDER BY y DESC) AS tab;

]]></screen>

But thisapproach is notguaranteed to workinall situations, and

it is not strictly SQL-conforming. A future version of PostgreSQL

might provide an additional feature to control the order in a

better-defined way (<literal>xmlagg(expr ORDER BY expr, expr,

...</literal>).

But thissyntax is notallowedinthe 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 abetter-defined way (<literal>xmlagg(expr ORDER BY expr, expr,

...)</literal>).

</para>

<para>