Module org.apache.jena.arq

Package org.apache.jena.sparql.algebra.optimize

Class TransformDistinctToReduced

java.lang.Object

org.apache.jena.sparql.algebra.TransformCopy

org.apache.jena.sparql.algebra.optimize.TransformDistinctToReduced

All Implemented Interfaces:

Transform

public class TransformDistinctToReduced
extends TransformCopy

Transforms generic DISTINCT plus ORDER BY combinations to REDUCED plus ORDER BY which typically gives better performance and memory consumption because engines have to keep less data in-memory to evaluate it.

As with most optimizations this is only applied when it is safe to do so. The criteria for being safe to do so are as follows:

• Uses both ORDER BY and DISTINCT on the same level of the query
• There is a fixed list of variables to project i.e. not SELECT *
• ORDER BY conditions cover all the projected variables prior to the use of any other variables

Related Optimizations

See also TransformOrderByDistinctApplication which is a better optimization for these kinds of queries but only applies to a more limited range of queries. Where possible that optimization is applied in preference to this one.

TransformTopN covers the case of DISTINCT plus ORDER BY where there is also a LIMIT. Where possible that optimization is applied in preference to either this or TransformOrderByDistinctApplication.

Field Summary

Fields inherited from class org.apache.jena.sparql.algebra.TransformCopy

COPY_ALWAYS, COPY_ONLY_ON_CHANGE

Constructor Summary

Constructors

Constructor	Description
TransformDistinctToReduced()

Method Summary

Modifier and Type	Method	Description
Op	transform(OpDistinct opDistinct, Op subOp)
Op	transform1(OpDistinct opDistinct, Op subOp)

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, transform

Methods inherited from class java.lang.Object

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

Constructor Details

TransformDistinctToReduced

public TransformDistinctToReduced()

Method Details

transform1

public Op transform1(OpDistinct opDistinct,
 Op subOp)

transform

public Op transform(OpDistinct opDistinct,
 Op subOp)

Specified by:

transform in interface Transform

Overrides:

transform in class TransformCopy