// override RexShuttle
 public RexNode visitCall(final RexCall call) {
   int i = reducibleExps.indexOf(call);
   if (i == -1) {
     return super.visitCall(call);
   }
   RexNode replacement = reducedValues.get(i);
   if (addCasts.get(i) && (replacement.getType() != call.getType())) {
     // Handle change from nullable to NOT NULL by claiming
     // that the result is still nullable, even though
     // we know it isn't.
     //
     // Also, we cannot reduce CAST('abc' AS VARCHAR(4)) to 'abc'.
     // If we make 'abc' of type VARCHAR(4), we may later encounter
     // the same expression in a ProjectRel's digest where it has
     // type VARCHAR(3), and that's wrong.
     replacement = rexBuilder.makeCast(call.getType(), replacement);
   }
   return replacement;
 }
  protected void executeImpl() throws Exception {
    SqlCall call = (SqlCall) subq;
    SqlSelect select = (SqlSelect) call.getOperands()[0];

    // Convert the SqlNode tree to a RelNode tree; we need to do this
    // here so the RelNode tree is associated with the new preparing
    // stmt.
    FarragoPreparingStmt preparingStmt = (FarragoPreparingStmt) getPreparingStmt();
    SqlValidator validator = preparingStmt.getSqlValidator();
    SqlToRelConverter sqlConverter = preparingStmt.getSqlToRelConverter(validator, preparingStmt);
    preparingStmt.setParentStmt(FennelRelUtil.getPreparingStmt(parentConverter.getCluster()));

    // Add to the new converter any subqueries that have already been
    // converted by the parent so we can avoid re-executing them
    sqlConverter.addConvertedNonCorrSubqs(parentConverter.getMapConvertedNonCorrSubqs());
    RelNode plan = sqlConverter.convertQuery(select, true, true);

    // The subquery cannot have dynamic parameters
    if (sqlConverter.getDynamicParamCount() > 0) {
      failed = true;
      return;
    }

    List<RexNode> exprs = new ArrayList<RexNode>();
    RelDataType resultType = null;

    if (!isExists) {
      // Non-EXISTS subqueries need to be converted to single-value
      // subqueries
      plan = sqlConverter.convertToSingleValueSubq(select, plan);

      // Create a dummy expression to store the type of the result.
      // When setting the type, derive the type based on what a
      // scalar subquery should return and create the type from the
      // type factory of the parent query.
      resultType = call.getOperator().deriveType(validator, validator.getFromScope(select), call);
      resultType = rexBuilder.getTypeFactory().copyType(resultType);
      exprs.add(rexBuilder.makeInputRef(resultType, 0));
    }

    plan = sqlConverter.decorrelate(select, plan);

    // If the subquery is part of an EXPLAIN PLAN statement, don't
    // execute the subquery, but instead just return a dynamic parameter
    // as a placeholder for the subquery result.  Otherwise, execute
    // the query to produce the constant expression.  Cast the expression
    // as needed so the type matches the expected result type.
    RexNode constExpr;
    if (isExplain) {
      if (isExists) {
        resultType = rexBuilder.getTypeFactory().createSqlType(SqlTypeName.BOOLEAN);
      }
      constExpr =
          rexBuilder.makeDynamicParam(
              resultType, parentConverter.getDynamicParamCountInExplain(true));
      results.add(constExpr);
    } else {
      executePlan(plan, exprs, isExists, false);
      if (!failed && !isExists) {
        constExpr = results.get(0);
        if (constExpr.getType() != resultType) {
          constExpr = rexBuilder.makeCast(resultType, constExpr);
          results.set(0, constExpr);
        }
      }
    }
  }