Apply binary search filter expressions directly on the block metadata…

… of `Index Scan`s (#1619)

With this PR, filter expressions that can be evaluated via binary search on a sorted input are directly evaluated on the block metadata of an IndexScan. For example in a query that contains `{ ?s ?p ?o FILTER (?o > 3)`} only the blocks of the full index scan (sorted by the object) are read from disk that according to their metadata might contain values `> 3`.

Currently this mechanism has the following limitations:
1. It can only be applied if the IndexScan directly is the child of the FILTER clause
2. It can only be applied to logical expressions (AND/OR/NOT) and to relational expressions (greater than, equal to, etc.) between a variable and a constant. Currently the constant can not yet be an IRI or Literal.

src/engine/Filter.cpp

@@ -27,7 +27,9 @@ Filter::Filter(QueryExecutionContext* qec,
                sparqlExpression::SparqlExpressionPimpl expression)
     : Operation(qec),
       _subtree(std::move(subtree)),
-      _expression{std::move(expression)} {}
+      _expression{std::move(expression)} {
+  setPrefilterExpressionForChildren();
+}
 
 // _____________________________________________________________________________
 string Filter::getCacheKeyImpl() const {
@@ -37,10 +39,22 @@ string Filter::getCacheKeyImpl() const {
   return std::move(os).str();
 }
 
+//______________________________________________________________________________
 string Filter::getDescriptor() const {
   return absl::StrCat("Filter ", _expression.getDescriptor());
 }
 
+//______________________________________________________________________________
+void Filter::setPrefilterExpressionForChildren() {
+  std::vector<PrefilterVariablePair> prefilterPairs =
+      _expression.getPrefilterExpressionForMetadata();
+  auto optNewSubTree = _subtree->setPrefilterGetUpdatedQueryExecutionTree(
+      std::move(prefilterPairs));
+  if (optNewSubTree.has_value()) {
+    _subtree = std::move(optNewSubTree.value());
+  }
+}
+
 // _____________________________________________________________________________
 ProtoResult Filter::computeResult(bool requestLaziness) {
   LOG(DEBUG) << "Getting sub-result for Filter result computation..." << endl;

src/engine/Filter.h

@@ -10,10 +10,11 @@
 
 #include "engine/Operation.h"
 #include "engine/QueryExecutionTree.h"
-#include "engine/sparqlExpressions/SparqlExpressionPimpl.h"
 #include "parser/ParsedQuery.h"
 
 class Filter : public Operation {
+  using PrefilterVariablePair = sparqlExpression::PrefilterExprVariablePair;
+
  private:
   std::shared_ptr<QueryExecutionTree> _subtree;
   sparqlExpression::SparqlExpressionPimpl _expression;
@@ -58,6 +59,14 @@ class Filter : public Operation {
     return _subtree->getVariableColumns();
   }
 
+  // The method is directly invoked with the construction of this `Filter`
+  // object. Its implementation retrieves <PrefilterExpression, Variable> pairs
+  // from the corresponding `SparqlExpression` and uses them to call
+  // `QueryExecutionTree::setPrefilterGetUpdatedQueryExecutionTree()` on the
+  // `subtree_`. If necessary the `QueryExecutionTree` for this
+  // entity will be updated.
+  void setPrefilterExpressionForChildren();
+
   ProtoResult computeResult(bool requestLaziness) override;
 
   // Perform the actual filter operation of the data provided.