Class TransformImplicitLeftJoin

  • All Implemented Interfaces:
    Transform

    public class TransformImplicitLeftJoin
    extends TransformCopy

    Optimizer for transforming implicit joins. These covers queries like the following:

     SELECT *
     WHERE
     {
       ?s a ?type1 .
       OPTIONAL
       {
         ?t a ?type2 .
         FILTER(?s = ?t)
       }
     }
     

    Clearly this is a trivial example but doing this optimization can have big performance gains since it can completely eliminate cross products that we would otherwise be required to evaluate. The optimization where applicable results in a query of the following form:

     SELECT *
     WHERE
     {
       ?s a ?type1 .
       OPTIONAL
       {
         ?s a ?type1 .
         BIND(?s AS ?t)
       }
     }
     

    This does not handle the simpler case of implicit joins where OPTIONAL is not involved, for that see TransformFilterImplicitJoin

    Applicability

    This optimization aims to eliminate implicit left joins of the form ?x = ?y or SAMETERM(?x, ?y), the latter can almost always be safely eliminated while the former may only be eliminated in the case where we can guarantee that at least one of the variables is a non-literal e.g. it occurs in the subject/predicate position. In the case where this is not true the optimization may not be made since we cannot assume that we can map value equality to term equality by making the optimization.