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

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
> 
>

Received on Saturday, 8 October 2022 11:33:25 UTC