Re: Variadic aggregates vs. project policy

On 2013-08-30 06:34:47 -0700, David Johnston wrote:
> Tom Lane-2 wrote
> >> I was one who sent a bug report - this error is not too dangerous, but it
> >> is hidden, and difficult to find, if you don't know what can be happen.
> >> Same as bug with plpgsql and SQL identifier collisions. If you
> >> understand,
> >> then you can protect self well and  simply. If not, then it is a magic
> >> error. So still I am thing so best solution is
> >
> >> a) a warning when detect ORDER BY in variadic aggregates
> >
> > Such a warning would never be tolerated by users, because it would appear
> > even when the query is perfectly correct.
> >
> >> b) disallow ORDER BY in variadic aggregates in classic syntax, and enable
> >> it only in WITHIN GROUP syntax where is safe ,
> >
> > And we're *not* inventing randomly different syntax for variadic
> > aggregates.  That ship sailed when we did it this way for regular
> > functions.
>
> In the example case the problem is that ORDER BY constant is a valid, if
> not-very-useful, construct.  Can we warn on this specific usage and thus
> mitigate many of the potential avenues of mis-use?

That doesn't help against something like »SELECT string_agg(somecol
ORDER BY bar, separator)« where separator is a column.

> If we alter syntax for mitigation purposes I'd want to consider requiring
> parentheses around the columns that belong to the ORDER BY instead of using
> the full extended syntax of WITHIN GROUP.

I think that ship has sailed. The syntax is there and it's not going
away. Requiring different syntaxes for variadic/nonvariadic usages is
going to be a way much bigger pitfall for users.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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