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Commit82eafab

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Improve docs about using ORDER BY to control aggregate input order.
David Johnston pointed out that the original text here had been obsoletedby SQL:2008, which allowed ORDER BY in subqueries. We could weaken thetext to describe ORDER-BY-in-subqueries as an optional SQL feature that'spossibly unportable; but then the exact same statements would apply tothe alternative it's being compared to (ORDER-BY-in-aggregate-calls).So really that would be pretty useless; let's just take out the sentenceentirely. Instead, point out the hazard that any extra processing in theupper query might cause the subquery output order to be destroyed.Discussion: <CAKFQuwbAX=iO9QbpN7_jr+BnUWm9FYX8WbEPUvG0p+nZhp6TZg@mail.gmail.com>
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‎doc/src/sgml/func.sgml‎

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@@ -13102,8 +13102,9 @@ SELECT count(*) FROM sometable;
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SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
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]]></screen>
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But this syntax is not allowed in the SQL standard, and is
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not portable to other database systems.
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Beware that this approach can fail if the outer query level contains
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additional processing, such as a join, because that might cause the
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subquery's output to be reordered before the aggregate is computed.
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</para>
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<para>

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