Class TransformFilterEquality

  • All Implemented Interfaces:
    Transform

    public class TransformFilterEquality
    extends TransformCopy
    A transform that aims to optimize queries where there is an equality constraint on a variable to speed up evaluation e.g
     SELECT *
     WHERE
     {
       ?s ?p ?o .
       FILTER(?s = <http://subject>)
     }
     
    Would transform to the following:
     SELECT *
     WHERE
     {
       <http://subject> ?p ?o .
       BIND(<http://subject> AS ?s)
     }
     

    Applicability

    This optimizer is conservative in that it only makes the optimization where the equality constraint is against a non-literal as otherwise substituting the value changes the query semantics because it switches from value equality to the more restrictive term equality. The optimization is safe for non-literals because for those value and term equality are equivalent (in fact value equality is defined to be term equality).

    There are also various nested algebra structures that can make the optimization unsafe and so it does not take place if any of those situations is detected.