LINQ到实体：

Question

我使用EF 4.4.20627.0与MySQL 5.6 LINQ查询性能优化，MySQL .net connector版本6.6.4

我有这样的方法，该方法生成的SQL是very very slow（需要超过1分钟）

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(); 
    } 

的generaed这样的SQL：

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 */ 

和

如果我移动order bylimit条款到支架，这将SQL very fast（成本低于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. */ 

是什么ProjectX和Extent1是什么意思？ 以及为什么实体框架不会将orderby * limit x,y置于真实查询之外？

sql很奇怪，绝对会让查询非常慢，我绝不会写这样的sql ......那么如何让EF生成一个CORRECT sql？

有什么建议吗？

Answer 1

只是一个快速猜测：Count（）和ToList（）都执行查询。首先执行ToList（），然后使用收到的列表获取元素的数量。

像这样：

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 myList = dataSrc.ToList(); //execute the query to an in-memory list 

    var cnt = myList.Count(); //get the count from the already exeuted query 
    SetPagerValues(pagenum, pagesize, cnt); 

    return myList; //return the list 
}