Package org.eclipse.rdf4j.query.algebra.evaluation.impl.evaluationsteps

Class LeftJoinQueryEvaluationStep

java.lang.Object

org.eclipse.rdf4j.query.algebra.evaluation.impl.evaluationsteps.LeftJoinQueryEvaluationStep

All Implemented Interfaces:

QueryEvaluationStep

public final class LeftJoinQueryEvaluationStep
extends Object
implements QueryEvaluationStep

Nested Class Summary

Nested classes/interfaces inherited from interface org.eclipse.rdf4j.query.algebra.evaluation.QueryEvaluationStep

QueryEvaluationStep.DelayedEvaluationIteration

Field Summary

Fields inherited from interface org.eclipse.rdf4j.query.algebra.evaluation.QueryEvaluationStep

EMPTY, EMPTY_ITERATION

Constructor Summary

Constructors

Constructor	Description
LeftJoinQueryEvaluationStep(QueryEvaluationStep right, QueryValueEvaluationStep condition, QueryEvaluationStep left, LeftJoin leftJoin, Set<String> optionalVars)

Method Summary

Modifier and Type	Method	Description
static QueryEvaluationStep	determineRightEvaluationStep(LeftJoin join, QueryEvaluationStep prepareRightArg, QueryValueEvaluationStep joinCondition, Set<String> scopeBindingNames)	This function determines the way the right-hand side is evaluated.
CloseableIteration<BindingSet>	evaluate(BindingSet bindings)
static QueryEvaluationStep	supply(EvaluationStrategy strategy, LeftJoin leftJoin, QueryEvaluationContext context)

Methods inherited from class java.lang.Object

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

Constructor Detail

LeftJoinQueryEvaluationStep

public LeftJoinQueryEvaluationStep(QueryEvaluationStep right,
                                   QueryValueEvaluationStep condition,
                                   QueryEvaluationStep left,
                                   LeftJoin leftJoin,
                                   Set<String> optionalVars)

Method Detail

supply

public static QueryEvaluationStep supply(EvaluationStrategy strategy,
                                         LeftJoin leftJoin,
                                         QueryEvaluationContext context)

evaluate

public CloseableIteration<BindingSet> evaluate(BindingSet bindings)

Specified by:

evaluate in interface QueryEvaluationStep

determineRightEvaluationStep

public static QueryEvaluationStep determineRightEvaluationStep(LeftJoin join,
                                                               QueryEvaluationStep prepareRightArg,
                                                               QueryValueEvaluationStep joinCondition,
                                                               Set<String> scopeBindingNames)

This function determines the way the right-hand side is evaluated. There are 3 options:

1. No join condition:
The right-hand side should just be joined with the left-hand side. No filtering is applied.

2. The join condition can be fully evaluated by the left-hand side:

 SELECT * WHERE {
        ?dist a dcat:Distribution .
  ?dist dc:license ?license .

  OPTIONAL {
        ?a dcat:distribution ?dist.

      FILTER(?license = )
        }
 }
 

In this case, pre-filtering can be applied. The right-hand side does not have to evaluated when the join condition evaluates to false.

3. The join condition needs right-hand side evaluation:

 SELECT * WHERE {
  ?dist a dcat:Distribution .

        OPTIONAL {
                ?a dcat:distribution ?dist .
                ?a dct:language $lang .

                FILTER(?lang = eu-lang:ENG)
        }
 }
 

In this case, the join condition can only be evaluated after the right-hand side is evaluated (post-filtering).