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* Licensed to the Apache Software Foundation (ASF) under one
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* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.hp.hpl.jena.sparql.algebra.optimize;
import com.hp.hpl.jena.sparql.algebra.Op ;
import com.hp.hpl.jena.sparql.algebra.TransformCopy ;
import com.hp.hpl.jena.sparql.algebra.op.OpConditional ;
import com.hp.hpl.jena.sparql.algebra.op.OpFilter ;
import com.hp.hpl.jena.sparql.algebra.op.OpJoin ;
import com.hp.hpl.jena.sparql.algebra.op.OpLeftJoin ;
import com.hp.hpl.jena.sparql.algebra.op.OpSequence ;
import com.hp.hpl.jena.sparql.algebra.op.OpTable ;
import com.hp.hpl.jena.sparql.engine.main.JoinClassifier ;
import com.hp.hpl.jena.sparql.engine.main.LeftJoinClassifier ;
import com.hp.hpl.jena.sparql.util.Context ;
/** Choose join strategy */
public class TransformJoinStrategy extends TransformCopy
{
private final Context context ;
// OpSequence - linear join
// OpCondition - linear left join
public TransformJoinStrategy(Context context)
{
this.context = context ;
}
@Override
public Op transform(OpJoin opJoin, Op left, Op right)
{
// Look one level in for any filters with out-of-scope variables.
boolean canDoLinear = JoinClassifier.isLinear(opJoin) ;
if ( canDoLinear )
{
if ( right instanceof OpTable )
{
// Safe to swap? Need to reclassify.
boolean b = JoinClassifier.isLinear(right, left) ;
if ( b )
{
// Swap left and right so start with a flow of concrete data.
Op tmp = left ;
left = right ;
right = tmp ;
}
}
// Streamed evaluation
return OpSequence.create(left, right) ;
}
// Can't do better.
return super.transform(opJoin, left,right) ;
}
//public Op transform(OpSequence opSequence, List<Op> elts)
@Override
public Op transform(OpLeftJoin opLeftJoin, Op opLeft, Op opRight)
{
// Test whether we can do an indexed substitute into the right if possible.
boolean canDoLinear = LeftJoinClassifier.isLinear(opLeftJoin) ;
if ( canDoLinear )
{
// Pass left into right for substitution before right side evaluation.
// In an indexed left join, the LHS bindings are visible to the
// RHS execution so the expression is evaluated by moving it to be
// a filter over the RHS pattern.
if (opLeftJoin.getExprs() != null )
opRight = OpFilter.filter(opLeftJoin.getExprs(), opRight) ;
return new OpConditional(opLeft, opRight) ;
}
// Not index-able.
return super.transform(opLeftJoin, opLeft, opRight) ;
}
}