/**
Copyright (C) SYSTAP, LLC DBA Blazegraph 2006-2016. All rights reserved.
Contact:
SYSTAP, LLC DBA Blazegraph
2501 Calvert ST NW #106
Washington, DC 20008
licenses@blazegraph.com
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/*
* Created on Nov 27, 2011
*/
package com.bigdata.rdf.sparql.ast.hints;
import com.bigdata.rdf.sparql.ast.ASTBase;
import com.bigdata.rdf.sparql.ast.NamedSubqueryRoot;
import com.bigdata.rdf.sparql.ast.QueryHints;
import com.bigdata.rdf.sparql.ast.QueryRoot;
import com.bigdata.rdf.sparql.ast.SubqueryRoot;
import com.bigdata.rdf.sparql.ast.eval.AST2BOpContext;
import com.bigdata.rdf.sparql.ast.optimizers.ASTSparql11SubqueryOptimizer;
/**
* Query hint indicating whether or not a Sub-Select should be transformed into
* a named subquery, lifting its evaluation out of the main body of the query
* and replacing the subquery with an INCLUDE. When <code>true</code>, the
* subquery will be lifted out. When <code>false</code>, the subquery will not
* be lifted unless other semantics require that it be lifted out regardless.
* This hint must be used with {@link QueryHintScope#SubQuery}.
* <p>
* For example, the following may be used to lift out the sub-select in which it
* appears into a {@link NamedSubqueryRoot}. The lifted expression will be
* executed exactly once.
*
* <pre>
* hint:SubQuery hint:runOnce "true" .
* </pre>
* <p>
* Note: This sets the {@link SubqueryRoot.Annotations#RUN_ONCE} AST annotation
* which is then interpreted by the {@link ASTSparql11SubqueryOptimizer}.
*/
final class RunOnceHint extends AbstractBooleanQueryHint {
protected RunOnceHint() {
super(QueryHints.RUN_ONCE, null/* default */);
}
@Override
public void handle(final AST2BOpContext context,
final QueryRoot queryRoot,
final QueryHintScope scope, final ASTBase op, final Boolean value) {
if (scope != QueryHintScope.SubQuery) {
throw new QueryHintException(scope, op,
getName(), value);
}
if (op instanceof SubqueryRoot) {
_setAnnotation(context, scope, op,
SubqueryRoot.Annotations.RUN_ONCE, value);
}
}
}