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
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.
COPY_ALWAYS, COPY_ONLY_ON_CHANGE
Constructor and Description |
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TransformImplicitLeftJoin() |
Modifier and Type | Method and Description |
---|---|
Op |
transform(OpLeftJoin opLeftJoin,
Op left,
Op right) |
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public Op transform(OpLeftJoin opLeftJoin, Op left, Op right)
transform
in interface Transform
transform
in class TransformCopy
Licenced under the Apache License, Version 2.0