private boolean addFilter(RexNode filter, Object[] filterValues) {

    if (filter.isA(SqlKind.EQUALS)) {

      final RexCall call = (RexCall) filter;

      RexNode left = call.getOperands().get(0);

      if (left.isA(SqlKind.CAST)) {

        left = ((RexCall) left).operands.get(0);

      }

      final RexNode right = call.getOperands().get(1);

      if (left instanceof RexInputRef

          && right instanceof RexLiteral) {

        final int index = ((RexInputRef) left).getIndex();

        if (filterValues[index] == null) {

          filterValues[index] = ((RexLiteral) right).getValue2().toString();

          return true;

        }

      }

    }

    return false;

  }