com.bigdata.rdf.sparql.ast.optimizers
Class ASTUnknownTermOptimizer

java.lang.Object
  com.bigdata.rdf.sparql.ast.optimizers.ASTUnknownTermOptimizer

All Implemented Interfaces:

IASTOptimizer

public class ASTUnknownTermOptimizer
extends Object
implements IASTOptimizer

Pruning rules for unknown IVs in statement patterns: If an optional join is known to fail, then remove the optional group in which it appears from the group (which could be an optional group, a join group, or a union). If a statement pattern contains an unknown term, a with this statement pattern will certainly fail. Thus the group in which the statement pattern appears (the parent) will also fail. Continue recursively up the parent hierarchy until we hit a UNION or an OPTIONAL parent. If we reach the root of the where clause for a subquery, then continue up the groups in which the subquery appears. If the parent is a UNION, then remove the child from the UNION. If a UNION has one child, then replace the UNION with the child. If a UNION is empty, then fail the group in which it fails (unions are not optional). These rules should be triggered if a join is known to fail, which includes the case of an unknown IV in a statement pattern as well GRAPH uri {} where uri is not a named graph.

 
 TODO From BigdataEvaluationStrategyImpl3#945
 
 Prunes the sop tree of optional join groups containing values
 not in the lexicon.
 
         sopTree = stb.pruneGroups(sopTree, groupsToPrune);
 
 
 If after pruning groups with unrecognized values we end up with a
 UNION with no subqueries, we can safely just return an empty
 iteration.
 
         if (SOp2BOpUtility.isEmptyUnion(sopTree.getRoot())) {
             return new EmptyIteration();
         }
 

and also if we encounter a value not in the lexicon, we can still continue with the query if the value is in either an optional tail or an optional join group (i.e. if it appears on the right side of a LeftJoin). We can also continue if the value is in a UNION. Otherwise we can stop evaluating right now.

                 } catch (UnrecognizedValueException ex) {
                     if (sop.getGroup() == SOpTreeBuilder.ROOT_GROUP_ID) {
                         throw new UnrecognizedValueException(ex);
                     } else {
                         groupsToPrune.add(sopTree.getGroup(sop.getGroup()));
                     }
                 }
 

ASTPruneUnknownTerms : If an unknown terms appears in a StatementPatternNode then we get to either fail the query or prune that part of the query. If it appears in an optional, then prune the optional. if it appears in union, the prune that part of the union. if it appears at the top-level then there are no solutions for that query. This is part of what BigdataEvaluationStrategyImpl3#toPredicate(final StatementPattern stmtPattern) is doing. Note that toVE() as called from that method will throw an UnknownValueException if the term is not known to the database. FIXME Isolate pruning logic since we need to use it in more than one place.

Constructor Summary

ASTUnknownTermOptimizer()

Method Summary

IQueryNode

optimize(AST2BOpContext context, IQueryNode queryNode, IBindingSet[] bindingSets) Optimize the AST.

Methods inherited from class java.lang.Object

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

Constructor Detail

ASTUnknownTermOptimizer

public ASTUnknownTermOptimizer()

Method Detail

optimize

public IQueryNode optimize(AST2BOpContext context,
                           IQueryNode queryNode,
                           IBindingSet[] bindingSets)

Description copied from interface: IASTOptimizer

Optimize the AST.

Specified by:

optimize in interface IASTOptimizer

Parameters:

context - The evaluation context.

queryNode - The AST (in).

Returns:

The optimized AST.