Re: Undefined results order with Order By/Projection/Distinct

Hi,
I noticed this issue some time ago too and tested that different engines
give different results.
I do not think this is a simple matter of wording (consider that ordering
by a variable that is not projected makes perfectly sense, the issue arises
only when also DISTINCT is used).

Miguel

On Sat, 8 Oct 2022, 13:34 Andy Seaborne, <andy@apache.org> wrote:

> Hi Pavel,
>
> Hmm - that text isn't good. The intent is, I guess, when
> distinct/project are same set of expressions as the orderby.
>
> I've recorded the issue
>
> https://www.w3.org/2013/sparql-errata#errata-query-20
>
>      Andy
>
> On 07/10/2022 15:38, Pavel Klinov wrote:
> > Hi all,
> >
> > Sorry if this is known but was a little surprise to me. Even though
> > both Projection and Distinct solution modifiers are required to
> > preserve the order of solutions imposed by Order By (see 18.5), one
> > can construct an example where the final query results would be
> > undefined:
> >
> > SELECT DISTINCT ?a {
> >      VALUES (?a ?b) { (1 1) (1 2) (2 3) (1 4) (2 5) }
> > }
> > ORDER BY DESC(?b)
> >
> > Solution modifiers are applied in the order of: Order By -> Projection
> > -> Distinct (15). So after the projection, the solution sequence is:
> > ?a -> 2, ?a -> 1, ?a -> 2, ?a -> 1, ?a -> 1.
> >
> > Now, the Distinct is only required to keep this order but it's free to
> > remove any of the duplicate ?a -> 2 or ?a -> 1 solutions. So the final
> > results could be either ?a -> 2, ?a -> 1 or ?a -> 1, ?a -> 2. Note
> > that both solution sequences preserve the Order By order!
> >
> > It's easy to make an extended example with LIMIT where the results
> > could be completely different based on how Distinct eliminates
> > duplicates. Given the role preservation requirement one can argue that
> > Distinct should always keep the first occurrence of each duplicate in
> > the input, but I don't think it's in the spec.
> >
> > Am I missing something?
> > Pavel
> >
> >
>
>