ADQL Erratum 2

[Marco Molinaro]

I prefer Mark's version.
But I'm ok with Markus's last suggestion too.

Marco

2017-06-06 12:50 GMT+02:00 Mark Taylor <M.B.Taylor at bristol.ac.uk>:
> On Tue, 30 May 2017, Markus Demleitner wrote:
>
>> However,
>>
>> > Now, I understand that different backends act differently, and that
>> > the usage of the seed value itself is quite confused/confusing,
>> > but if we go too far, we may slip out of the context for an erratum.
>>
>> goes against ADQL's purpose of having an interoperable syntax and
>> semantics for an SQL dialect.  Worse, at least some backends (Gregory
>> tried it with H2; my former implementation did about the same) have
>> utterly useless and dangerous behaviour: they seed the RNG in each
>> row, which means that all calls to random() yield the same value(s)
>> in all rows.
>>
>> Given that, I think we have to warn people that using the argument
>> will very likely not do what at least I would expect: for each
>> occurrence of random(n) generate an independent, reproducable
>> sequence
>>
>>   row-index -> pseudo random number
>>
>> I'm not even sure if there is any DB engine implementing this at all.
>>
>> Explaining this in this many words is, I'd say, a bit too much for
>> this table.  So, here's my second attempt to provide a text for the
>> table:
>>
>>   rand([x]) -- Returns a random value between 0.0 and 1.0.  The
>>   optional argument, originally intended to provide a random seed,
>>   should not be used.  Behaviour for the function with an argument is
>>   undefined.  Query writers should not use it.
>>
>> Opinions?
>
> I'd be OK with that, though it sounds slightly heavy handed.
> Some ADQL implementation/DBMS might do something useful with x,
> and if a query writer understands that is the case they might
> legitimately want to use it.  Suggest instead:
>
>     rand([x]) -- Returns a random value between 0.0 and 1.0.  The
>     optional argument, originally intended to provide a random seed,
>     has undefined semantics.  Query writers are advised to use the
>     form without the argument.
>
> But, if Markus's suggested text is used instead (e.g. because such
> implementation-aware ADQL query construction is deemed evil) it's OK.
>
> Mark
>
> --
> Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
> m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/