On 11/06/2010 12:06 PM, Rob Vesse wrote:
Hi
I was looking at the MINUS syntax which as I understand it will now be
in SPARQL 1.1 and was wondering about how best to optimise it in my
SPARQL implementation.
As I understand it a MINUS pattern like the following has no effect
since the two BGPs are disjoint and thus no solutions can possibly be
eliminated:
SELECT * { ?s ?p ?o MINUS { ?x ?y ?z } }
A naive Algebra transform of this query would I presume be MINUS( BGP( {
?s ?p ?o } ), BGP( { ?x ?y ?z } ) )
But since in any such case the MINUS can surely be eliminated since it
will have no effect so an alternative algebra transform would just be
BGP( { ?s ?p ?o } )
Is this an acceptable and sensible way to optimise MINUS implementation
or have I missed something vital in my interpretation of the MINUS pattern?
Hi Rob,
"Acceptable" is anything that achieves the correct answers. The spec
will define what the right answers are, not how to get them, and
certainly nor the best way to get them. Anyway you spot to do it
efficiently is fair game. That one does get the right answers.
Personally, I have split this into two steps: generate the exact algebra
from the query first, then see if there are any improvements to the
algebra that can be made (including changing to other algebra operators
not in the SPARQL set).
Andy
