linq to entity: linq query performance optimization

问题

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?

回答1:

Just a quick guess: The Count() and ToList() both execute the query. Do the ToList() first, and use the received list to get the count of elements.

Like so:

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
}

来源：https://stackoverflow.com/questions/14191998/linq-to-entity-linq-query-performance-optimization

标签

mysql

sql

entity-framework

linq-to-entities