mysql5.5.25 源码阅读---rename处理流程
在rename的处理过程中，frm ibd文件都是直接改名，并没有将原表数据导入新表的过程，所以速度还是很快。
drop 比较大的表时可以利用下这个特点
可以先rename，
然后结合这个方法异步进行http://www.mysqlops.com/2011/05/18/mysql%E5%88%A0%E9%99%A4%E5%A4%A7%E8%A1%A8%E6%9B%B4%E5%BF%AB%E7%9A%84drop-table%E5%8A%9E%E6%B3%95.html


sql_rename.cc::do_rename
  build_table_filename(name, sizeof(name) - 1,new_db, new_alias, reg_ext, 0);
 
  build_table_filename(name, sizeof(name) - 1,ren_table->db,old_alias,reg_ext, 0);
  switch (frm_type)
  {
    case FRMTYPE_TABLE:
    {
       ## rename table
       if (!(rc= mysql_rename_table(ha_resolve_by_legacy_type(
                                    thd,table_type),
                                    ren_table->db,old_alias,
                                    new_db, new_alias
                                    , 0)))
       {
          ## 改触发器引用的原表名
          if ((rc= Table_triggers_list::change_table_name(thd, ren_table->db,
                                                          old_alias,
                                                          ren_table->table_name,
                                                          new_db,
                                                          new_alias)))
          {
            ## 失败进行回滚
            (void) mysql_rename_table(ha_resolve_by_legacy_type(thd,table_type),
                                   new_d