ImplicitJoinOptimiser Class

An Algebra Optimiser which implements the Implicit Join optimisation.

Inheritance Hierarchy

SystemObject
VDS.RDF.Query.OptimisationImplicitJoinOptimiser

Namespace: VDS.RDF.Query.Optimisation
Assembly: dotNetRDF (in dotNetRDF.dll) Version:

Syntax

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public class ImplicitJoinOptimiser : IAlgebraOptimiser

Public Class ImplicitJoinOptimiser
	Implements IAlgebraOptimiser

The ImplicitJoinOptimiser type exposes the following members.

Constructors

	Name	Description
	ImplicitJoinOptimiser	Initializes a new instance of the ImplicitJoinOptimiser class

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Methods

	Name	Description
	Equals	Determines whether the specified object is equal to the current object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
	GetHashCode	Serves as the default hash function. (Inherited from Object.)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	IsApplicable(SparqlQuery)	Returns that this optimiser is applicable to all queries.
	IsApplicable(SparqlUpdateCommandSet)	Returns that this optimiser is applicable to all updates.
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	Optimise	Optimises the Algebra to use implict joins where applicable.
	ToString	Returns a string that represents the current object. (Inherited from Object.)

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Remarks

An implict join is implied by a query like the following:

            SELECT *
            WHERE
            {
              ?x a ?type .
              ?y a ?type .
              FILTER (?x = ?y) .
            }

Such queries can be very expensive to calculate, the implict join optimisation attempts to substitute one variable for the other and use a BIND to ensure both variables are visible outside of the graph pattern affected i.e. the resulting query looks like the following:

            SELECT *
            WHERE
            {
              ?x a ?type .
              ?x a ?type .
              BIND (?x AS ?y)
            }

Under normal circumstances this optimisation is only used when the implict join is denoted by a SAMETERM expression or the optimiser is sure the variables don't represent literals (they never occur in the Object position) since when value equality is involved substituing one variable for another changes the semantics of the query and may lead to unexpected results. Since this optimisation may offer big performance benefits for some queries (at the cost of potentially incorrect results) this form of the optimisation is allowed when you set UnsafeOptimisation to true.

This optimiser is also capable of generating special algebra to deal with the case where there is an implicit join but the substitution based optimisation does not apply because variables cannot be substituted into the inner algebra, in this case a FilteredProduct is generated instead.

Reference

VDS.RDF.Query.Optimisation Namespace