public static final RelOptRule[] RULES = {

    MongoSortRule.INSTANCE,

    MongoFilterRule.INSTANCE,

    MongoProjectRule.INSTANCE,

    MongoAggregateRule.INSTANCE,

  };



  static String isItem(RexCall call) {

    if (call.getOperator() != SqlStdOperatorTable.ITEM) {

      return null;

    }

    final RexNode op0 = call.operands.get(0);

    final RexNode op1 = call.operands.get(1);

    if (op0 instanceof RexInputRef

        && ((RexInputRef) op0).getIndex() == 0

        && op1 instanceof RexLiteral

        && ((RexLiteral) op1).getValue2() instanceof String) {

      return (String) ((RexLiteral) op1).getValue2();

    }

    return null;

  }