Linq for an object: optimizing linq query performance

Question

I am using EF 4.4.20627.0with MySQL 5.6, MySQL .net connectorversion 6.6.4

I have a method that generated sql: very very slow(it takes more than 1 minute)

    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();
    }

generalized 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 moved the proposal order by, and limitin the bracket, the sql is very fast(costs less 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 do ProjectXu mean Extent1? and why does entity-framework not place orderby * limit x,youtside the real request?

sql is strange and definitely makes the query very slow, I will never write sql like this ... So, how to make EF generate CORRECTsql ??

any suggestions?

Answer 1

: Count() ToList() . ToList() , .

:

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
}