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help / color / mirror / Atom feedFrom: David G. Johnston <[email protected]>
To: Tom Lane <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Is this example regarding aggregates sourced by subquery correct?
Date: Sat, 21 May 2016 13:13:38 -0400
Message-ID: <CAKFQuwYD49-=7XL=i-F7VMP3o5Kjwo2a9v6LQVn_5RV4DDzN-A@mail.gmail.com> (raw)
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On Saturday, May 21, 2016, Tom Lane <[email protected]> wrote:
> "David G. Johnston" <[email protected] <javascript:;>> writes:
> > Based upon what you've said I would soften it a bit. Given my own
> > experience I'd probably point out what is now obvious to me - that the
> > allowance of the ORDER BY clause is implementation specific. But I'd be
> > fine chalking that up to an anomalous reading.
>
> > Something like:
>
> > "But permitting the sub-query's ORDER BY was only upgraded to optional in
> > SQL:2008 and thus this syntax poses a portability hazard."
>
> After further thought I realized that this gripe applies just as much
> to the alternative we're comparing this to, ie, putting ORDER BY into
> the aggregate call. (I've not looked up whether the two features were
> introduced in exactly the same SQL version, but I am pretty sure they
> are both post-SQL99.) So we might as well just take it out. What we
> could usefully do instead is explain exactly what's dangerous about
> using a subquery ORDER BY in this way. So I changed it to
>
> Beware that this approach can fail if the outer query level contains
> additional processing, such as a join, because that might cause the
> subquery's output to be reordered before the aggregate is computed.
>
>
That works. There's only so much portability warning that is useful and we
should focus on better informing how postgreSQL functions.
Thanks.
David J.
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Subject: Re: Is this example regarding aggregates sourced by subquery correct?
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