@Override
public List<List<LeafNodePlan>> split(List<LeafNodePlan> initialPlans) {
// We determine the maximum number of possible merge joins!
final HashMap<Item, LinkedList<LeafNodePlan>> mergeJoins =
new HashMap<Item, LinkedList<LeafNodePlan>>();
for (final LeafNodePlan tp : initialPlans) {
for (final Variable var : tp.getVariables()) {
// just the group the leaf nodes with common variables together (this group can be surely
// joined only with merge joins)
LinkedList<LeafNodePlan> lltp = mergeJoins.get(var);
if (lltp == null) {
lltp = new LinkedList<LeafNodePlan>();
}
lltp.add(tp);
mergeJoins.put(var, lltp);
}
}
// determine the group the most possible merge joins
int maxMergeJoins = 0;
for (final LinkedList<LeafNodePlan> lltp : mergeJoins.values()) {
if (lltp.size() > maxMergeJoins) {
maxMergeJoins = lltp.size();
}
}
// avoid the trivial cases (only few merge joins or the whole leaf nodes form a star-shaped
// join)
if (maxMergeJoins > 2 && maxMergeJoins < initialPlans.size()) {
final LinkedList<LeafNodePlan> otherSubgraph = new LinkedList<LeafNodePlan>();
for (final LinkedList<LeafNodePlan> lltp : mergeJoins.values()) {
if (lltp.size() == maxMergeJoins) {
// determine the other leaf nodes, which are not contained in the subgraph with maximum
// number of merge joins!
for (LeafNodePlan lfp : initialPlans) {
if (!lltp.contains(lfp)) {
otherSubgraph.add(lfp);
}
}
// now check if otherSubGraph contains a cartesian product, which would be unacceptable!
List<List<LeafNodePlan>> cartesianProduct2 =
SplitCartesianProduct.cartesianProducts(otherSubgraph);
if (cartesianProduct2.size() <= 1) {
// no cartesian product detected => acceptable splitting!
List<List<LeafNodePlan>> result = new LinkedList<List<LeafNodePlan>>();
result.add(lltp);
result.add(otherSubgraph);
return result;
}
}
}
}
// this strategy did not work!
List<List<LeafNodePlan>> result = new LinkedList<List<LeafNodePlan>>();
result.add(initialPlans);
return result;
}
@Override
protected BasicIndexScan getIndex(
final LeafNodePlan plan,
final BasicIndexScan indexScan,
final Collection<Variable> sortCriterium,
final Map<Variable, Literal> minima,
final Map<Variable, Literal> maxima) {
BasicIndexScan index1 =
new MemoryIndexScan(
(OperatorIDTuple) null,
plan.getTriplePatterns(),
indexScan.getGraphConstraint(),
indexScan.getRoot());
index1.setIntersectionVariables(plan.getVariables());
index1.setUnionVariables(plan.getVariables());
return index1;
}