code: func (ts *TiDBStatement) Close() error {
  	//TODO close at tidb level
  	err := ts.ctx.session.DropPreparedStmt(ts.id)
  	if err != nil {
  		return err
  	}
  	delete(ts.ctx.stmts, int(ts.id))
  
  	// close ResultSet associated with this statement
  	if ts.rs != nil {
  		terror.Call(ts.rs.Close)
  	}
  	return nil
  }
docs: // Close implements PreparedStatement Close method.