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*
* 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,
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* See the License for the specific language governing permissions and
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package org.apache.jena.sparql.algebra.optimize;
import java.util.Collection ;
import java.util.HashSet;
import java.util.Set;
import org.apache.jena.query.SortCondition ;
import org.apache.jena.sparql.algebra.Op ;
import org.apache.jena.sparql.algebra.OpVars ;
import org.apache.jena.sparql.algebra.TransformCopy ;
import org.apache.jena.sparql.algebra.op.OpDistinct ;
import org.apache.jena.sparql.algebra.op.OpOrder ;
import org.apache.jena.sparql.algebra.op.OpProject ;
import org.apache.jena.sparql.algebra.op.OpReduced ;
import org.apache.jena.sparql.core.Var ;
/**
* <p>
* Transforms generic {@code DISTINCT} plus {@code ORDER BY} combinations to
* {@code REDUCED} plus {@code ORDER BY} which typically gives better
* performance and memory consumption because engines have to keep less data
* in-memory to evaluate it.
* </p>
* <p>
* 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:
* </p>
* <ul>
* <li>Uses both {@code ORDER BY} and {@code DISTINCT} on the same level of the
* query</li>
* <li>There is a fixed list of variables to project i.e. not {@code SELECT *}</li>
* <li>{@code ORDER BY} conditions cover all the projected variables prior to
* the use of any other variables</li>
* </ul>
* <h3>Related Optimizations</h3>
* <p>
* See also {@link 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.
* </p>
* <p>
* {@link TransformTopN} covers the case of {@code DISTINCT} plus
* {@code ORDER BY} where there is also a {@code LIMIT}. Where possible that
* optimization is applied in preference to either this or
* {@link TransformOrderByDistinctApplication}.
* </p>
*
*/
public class TransformDistinctToReduced extends TransformCopy {
public TransformDistinctToReduced() {}
// Best is this is after TransformTopN but they are order independent
// TopN of "reduced or distinct of order" is handled.
//@Override
public Op transform1(OpDistinct opDistinct, Op subOp) {
if (subOp instanceof OpProject) {
OpProject opProject = (OpProject) subOp;
if (opProject.getSubOp() instanceof OpOrder) {
OpOrder opOrder = (OpOrder) opProject.getSubOp();
Set<Var> projectVars = new HashSet<>(opProject.getVars()) ;
if (isSafe(projectVars, opOrder)) {
return OpReduced.create(subOp);
}
}
}
return super.transform(opDistinct, subOp);
}
@Override
public Op transform(OpDistinct opDistinct, Op subOp) {
OpOrder opOrder = null ;
Set<Var> projectVars = null ;
/* SELECT DISTINCT * {} ORDER BY
* giving an alegbra expression of the form:
* (distinct
* (order
*/
if (subOp instanceof OpOrder) {
opOrder = (OpOrder) subOp;
projectVars = OpVars.visibleVars(subOp) ;
} else if (subOp instanceof OpProject) {
OpProject opProject = (OpProject) subOp;
if (opProject.getSubOp() instanceof OpOrder) {
projectVars = new HashSet<>(opProject.getVars()) ;
opOrder = (OpOrder) opProject.getSubOp();
}
}
if ( projectVars == null )
return super.transform(opDistinct, subOp) ;
if (isSafe(projectVars, opOrder))
return OpReduced.create(subOp);
return super.transform(opDistinct, subOp);
}
protected boolean isSafe(Set<Var> projectVars, OpOrder opOrder) {
Set<Var> seenVars = new HashSet<>();
// For the optimization to be safe all project variables must appear in
// the ordering prior to any unprojected variables
// Ordering by expressions is fine provided they use only projected
// variables
boolean ok = true;
for (SortCondition cond : opOrder.getConditions()) {
if (!isValidSortCondition(cond, projectVars, seenVars)) {
ok = false;
break;
}
// As soon as we've seen all variables we know this is safe and any
// further sort conditions are irrelevant
if (seenVars.size() == projectVars.size())
return true ;
}
// The projects vars must all have been seen.
return (seenVars.size() == projectVars.size()) ;
}
/**
* Determines whether a sort condition is valid in terms of this optimizer
*
* @param cond
* Sort Condition
* @param projectVars
* Project Variables
* @return True if valid, false otherwise
*/
private boolean isValidSortCondition(SortCondition cond, Collection<Var> projectVars, Set<Var> seenVars) {
if (cond.getExpression().isVariable()) {
if (projectVars.contains(cond.getExpression().asVar())) {
seenVars.add(cond.getExpression().asVar());
return true;
}
return false;
} else {
for (Var v : cond.getExpression().getVarsMentioned()) {
if (!projectVars.contains(v))
return false;
seenVars.add(v);
}
return true;
}
}
}