Fix collisions of internally added variables for subqueries (#1848)

This fixes a bug where names of internal variables were wrongly reused between the body of a subquery (e.g. in the implementation of anonymous nodes (`[]`), and for internal aliases needed for `ORDER BY(expression)` or `GROUP BY(expression)`.
Fixes #1561

src/parser/ParsedQuery.h

@@ -173,16 +173,6 @@ class ParsedQuery {
     return _rootGraphPattern._graphPatterns;
   }
 
-  // TODO<joka921> This is currently necessary because of the missing scoping of
-  // subqueries
-  [[nodiscard]] int64_t getNumInternalVariables() const {
-    return numInternalVariables_;
-  }
-
-  void setNumInternalVariables(int64_t numInternalVariables) {
-    numInternalVariables_ = numInternalVariables;
-  }
-
  private:
   // Add a BIND clause to the body of the query. The second argument determines
   // whether the `targetVariable` will be part of the visible variables that are

src/parser/sparqlParser/SparqlQleverVisitor.cpp

@@ -1276,9 +1276,7 @@ Visitor::SubQueryAndMaybeValues Visitor::visit(Parser::SubSelectContext* ctx) {
   ParsedQuery& query = parsedQuery_;
   query._clause = visit(ctx->selectClause());
   visitWhereClause(ctx->whereClause(), query);
-  query.setNumInternalVariables(numInternalVariables_);
   query.addSolutionModifiers(visit(ctx->solutionModifier()));
-  numInternalVariables_ = query.getNumInternalVariables();
   auto values = visit(ctx->valuesClause());
   // Variables that are selected in this query are visible in the parent query.
   for (const auto& variable : query.selectClause().getSelectedVariables()) {

src/parser/sparqlParser/SparqlQleverVisitor.h

@@ -73,7 +73,6 @@ class SparqlQleverVisitor {
 
  private:
   size_t _blankNodeCounter = 0;
-  int64_t numInternalVariables_ = 0;
   int64_t numGraphPatterns_ = 0;
   // The visible variables in the order in which they are encountered in the
   // query. This may contain duplicates. A variable is added via

test/QueryPlannerTest.cpp

@@ -2982,6 +2982,16 @@ TEST(QueryPlanner, Exists) {
       filter);
 }
 
+// _____________________________________________________________________________
+TEST(QueryPlanner, ensureGeneratedInternalVariablesDontClash) {
+  h::expect("SELECT * { SELECT ?s { ?s <a> [] } ORDER BY RAND() }",
+            h::OrderBy({std::pair{Var{"?_QLever_internal_variable_1"},
+                                  OrderBy::AscOrDesc::Asc}},
+                       h::Bind(h::IndexScanFromStrings(
+                                   "?s", "<a>", "?_QLever_internal_variable_0"),
+                               "RAND()", Var{"?_QLever_internal_variable_1"})));
+}
+
 // _____________________________________________________________________________
 TEST(QueryPlanner, FilterOnNeutralElement) {
   h::expect("SELECT * { FILTER(false) }",