linq to entity: linq query performance optimization

i'm using EF 4.4.20627.0 with MySQL 5.6, MySQL .net connector version 6.6.4

i have a method like this, which generated sql is very very slow(needs more than 1 min)

    private List<TNews> GetPagedNews(int pagenum, int pagesize,
        AdvSearcherArgs advcArgs, string keyword)
    {
        var dataSrc = _dbRawDataContext.TNews.Where(x => x.Id>0);
        if (!string.IsNullOrWhiteSpace(advcArgs.PMAC))
        {
            dataSrc = dataSrc.Where(m => m.Pmac == advcArgs.PMAC);
        }
        if (!string.IsNullOrWhiteSpace(advcArgs.BegineDate))
        {
            var begin = Convertion.ToDate(advcArgs.BegineDate);
            var end = Convertion.ToDate(advcArgs.EndDate);

            dataSrc = dataSrc.Where(m => m.PmacDT >=begin && m.PmacDT<end);
        }
        dataSrc = dataSrc.OrderByDescending(n => n.PmacDT).Skip(pagenum * pagesize).
          Take(pagesize);

        var cnt = dataSrc.Count();
        SetPagerValues(pagenum, pagesize, cnt);

        return dataSrc.ToList();
    }

the generaed sql like this:

SELECT
`Project1`.*
FROM 
(
   SELECT
   `Extent1`.*
   FROM `tnews` AS `Extent1`
   WHERE (`Extent1`.`Id` > 0) 
   AND ((`Extent1`.`PmacDT` >= '2013-01-01 00:00:00 ') AND          
        (`Extent1`.`PmacDT` < '2013-01-07 00:00:00 '))
) AS `Project1`
 ORDER BY 
`Project1`.`PmacDT` DESC LIMIT 0,20
/* Affected rows: 0  Found rows: 20  Warnings: 0  Duration for 1 query: 00:01:30 */

if i move order by and limit clause into the brace, this sql will be very fast(costs less than 1 sec):

SELECT
`Project1`.*
FROM 
(
   SELECT
   `Extent1`.*
   FROM `tnews` AS `Extent1`
   WHERE (`Extent1`.`Id` > 0) 
   AND ((`Extent1`.`PmacDT` >= '2013-01-01 00:00:00 ') AND          
        (`Extent1`.`PmacDT` < '2013-01-07 00:00:00 '))
   ORDER BY 
   `PmacDT` DESC LIMIT 0,20
) AS `Project1`
 /* Affected rows: 0  Found rows: 20  Warnings: 0  Duration for 1 query: 0.000 sec. */

what does ProjectX and Extent1 mean? and why entity-framework don't put the orderby * limit x,y outside the real query??

the sql is weird and definitely make the query very slow, i will never write sql like that...So how to make EF generate a CORRECT sql??

any suggestions?