D('category')执行时．

SQL: SHOW COLUMNS FROM `yzj_category` [ RunTime:0.0170s ]
SQL: SHOW COLUMNS FROM `yzj_category` [ RunTime:0.0140s ]

M('category')执行时．

SQL: SHOW COLUMNS FROM `yzj_category` [ RunTime:0.0180s ]

为什么？

2016-04-20

查看代码发现如果先Ｍ('category'),再D('category')，就会执行两次．

SQL: SHOW COLUMNS FROM `yzj_category` [ RunTime:0.0170s ]
SQL: SHOW COLUMNS FROM `yzj_category` [ RunTime:0.0140s ]

是编写代码时的规则问题．