com.hp.hpl.jena.sparql.algebra.optimize
Class TransformImplicitLeftJoin

java.lang.Object
  com.hp.hpl.jena.sparql.algebra.TransformCopy
      com.hp.hpl.jena.sparql.algebra.optimize.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.

Field Summary

Fields inherited from class com.hp.hpl.jena.sparql.algebra.TransformCopy

COPY_ALWAYS, COPY_ONLY_ON_CHANGE

Constructor Summary

TransformImplicitLeftJoin()

Method Summary

Op	transform(OpLeftJoin opLeftJoin, Op left, Op right)

Methods inherited from class com.hp.hpl.jena.sparql.algebra.TransformCopy

transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform, transform

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

TransformImplicitLeftJoin

public TransformImplicitLeftJoin()

Method Detail

transform

public Op transform(OpLeftJoin opLeftJoin,
                    Op left,
                    Op right)

Specified by:

transform in interface Transform

Overrides:

transform in class TransformCopy