var query = from a in _wfoReceiveFile.GetAll().Where(p=>p.CreateAccountId == loginId)
join b in _wfInstance.GetAll() on a.Id equals b.EntityId into temp
from b in temp.DefaultIfEmpty()
join d in _sysUnit.GetAll() on a.SendUnitId equals d.Id

select new ReceiveFileListOutput
{
 。。。。。。。。。。。。。。。
};

上面的linq数据量稍微大点，直接连接查询超时， 稍微改一下：

var query = from a in _wfoReceiveFile.GetAll().Where(p=>p.CreateAccountId == loginId)

join d in _sysUnit.GetAll() on a.SendUnitId equals d.Id

join b in _wfInstance.GetAll() on a.Id equals b.EntityId into temp

from b in temp.DefaultIfEmpty()

select new ReceiveFileListOutput

{
 。。。。。。。。。。。。。。。
};

速度马上上来了， 不知道啥原理， 就把红色那行往上移动了