- From: Andy Seaborne <andy.seaborne@epimorphics.com>
- Date: Sun, 18 Mar 2012 16:46:35 +0000
- To: SPARQL Working Group <public-rdf-dawg@w3.org>

If we have some forms that are distinct and some counting, then we need an account for what happens when they are mixed. The obvious (to me) way is that it simply follows the combination of operators. Distinct operators only yield distinct results even if used over a path expression which may have duplicates; Counting operators that combine distinct sub-operators may cause duplicates but don't change the sub-operators. Support for these examples, assume | is not distinct and * is distinct: Example 1: (:a|:b)+ is distinct. Example 2: a+|b+ is not distinct. Data: :x1 :a :x2 . :x1 :b :x2 . { :x1 :a+ ?X } ?X => ?X = :x2 { :x1 :b+ ?X } ?X => ?X = :x2 Then { :x1 (:a|:b)+ ?X } ?X = :x2 But { :x1 :a+|:b+ ?X } => { :x1 :b+ ?X } UNION { :x1 :b+ ?X } => ?X = :x2 and ?X = :x2 In implementation terms, it just works out by combination of subparts. (An optimization includes a one bit flag to say if evaluating a form inside a context that will be distinct so the form only needs to be distinct and can otherwise be loose extra cardinality for efficiency.) Andy

Received on Sunday, 18 March 2012 16:47:07 UTC