com.hp.hpl.jena.sparql.algebra.optimize
Class TransformOrderByDistinctApplication

java.lang.Object
  com.hp.hpl.jena.sparql.algebra.TransformCopy
      com.hp.hpl.jena.sparql.algebra.optimize.TransformOrderByDistinctApplication

All Implemented Interfaces:

Transform

public class TransformOrderByDistinctApplication
extends TransformCopy

Improved optimization for ORDER BY plus DISTINCT or REDUCED combinations, see JENA-441 for original proposal and discussion.

This optimization is enabled by default as with most ARQ optimizations and may be disabled by setting the symbol ARQ.optOrderByDistinctApplication to false.

Optimization Applicability

This is a limited optimization that applies in the case where you have a query that meets the following conditions:

• Uses both ORDER BY and DISTINCT or REDUCED on the same level of the query
• There is a fixed list of variables to project i.e. not SELECT *
• ORDER BY conditions only use variables present in the project list

Essentially this takes algebras of the following form:

 (distinct 
   (project (?var) 
     (order (?var) 
       ... )))
 

And produces algebra of the following form:

 (order (?var)
   (distinct 
     (project (?var) 
       ... )))
 

In the general case this in unsafe because it would change the semantics of the query since ORDER BY can access variables that are not projected. However if the conditions outlined are met then this optimization is safe, the algebras will be semantically equivalent and the resulting form likely significantly more performant, of course YMMV depending on how much data you are querying.

Field Summary

Fields inherited from class com.hp.hpl.jena.sparql.algebra.TransformCopy

COPY_ALWAYS, COPY_ONLY_ON_CHANGE

Constructor Summary

TransformOrderByDistinctApplication()

Method Summary

Op	transform(OpDistinct opDistinct, Op subOp)
Op	transform(OpReduced opReduced, Op subOp)

Methods inherited from class com.hp.hpl.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

Methods inherited from class java.lang.Object

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

Constructor Detail

TransformOrderByDistinctApplication

public TransformOrderByDistinctApplication()

Method Detail

transform

public Op transform(OpDistinct opDistinct,
                    Op subOp)

Specified by:

transform in interface Transform

Overrides:

transform in class TransformCopy

transform

public Op transform(OpReduced opReduced,
                    Op subOp)

Specified by:

transform in interface Transform

Overrides:

transform in class TransformCopy