mybatis association select 性能分析

mybatis中，association select性能分析

<resultMap id="UserAndRoleResultMap" type="com.example.shiro_demo.entity.UserAndRole">
    <association property="sysUser" javaType="com.example.shiro_demo.entity.SysUser"
                 column="user_id"
                 select="com.example.shiro_demo.dao.SysUserDAO.selectByPrimaryKey">
    </association>
    <association property="sysRole" javaType="com.example.shiro_demo.entity.SysRole"
                 column="role_id"
                  select="com.example.shiro_demo.dao.SysRoleDAO.selectByPrimaryKey">
    </association>
  </resultMap>
  <select id="selectAll" resultMap="UserAndRoleResultMap">
    select * from sys_user_role
  </select>

实际上使用这种方式，相当于查询了三次，只是代码层次的联表查询，甚至不算是联表查询，结构上重用了代码而已。 如果注重性能建议还是使用下面这种方式

<resultMap id="UserAndRoleResultMap" type="com.example.shiro_demo.entity.UserAndRole">
    <association property="sysUser" javaType="com.example.shiro_demo.entity.SysUser">
      <id column="user_id" jdbcType="BIGINT" property="userId" />
      <result column="user_name" jdbcType="VARCHAR" property="userName" />
      <result column="full_name" jdbcType="VARCHAR" property="fullName" />
      <result column="password" jdbcType="VARCHAR" property="password" />
      <result column="salt" jdbcType="VARCHAR" property="salt" />
    </association>
    <association property="sysRole" javaType="com.example.shiro_demo.entity.SysRole">
      <id column="role_id" jdbcType="BIGINT" property="roleId" />
      <result column="role_name" jdbcType="VARCHAR" property="roleName" />
    </association>
  </resultMap>

  <select id="selectAll" resultMap="UserAndRoleResultMap">
    select * from sys_user_role as ur join sys_user as u on ur.user_id = u.user_id join sys_role as r on r.role_id = ur.role_id
  </select>