Class TransformImplicitLeftJoin
 All Implemented Interfaces:
Transform
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 nonliteral 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 org.apache.jena.sparql.algebra.TransformCopy
COPY_ALWAYS, COPY_ONLY_ON_CHANGE

Constructor Summary

Method Summary
Methods inherited from class org.apache.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

Constructor Details

TransformImplicitLeftJoin
public TransformImplicitLeftJoin()


Method Details

transform
 Specified by:
transform
in interfaceTransform
 Overrides:
transform
in classTransformCopy
