Question

EntityModel定义为： 人员有一个指向国家/地区的链接

在LinqPad中执行此代码时，我发现第一个查询中生成的SQL未优化（返回所有字段）？我在这里错过了什么或做错了什么？

查询1 LINQ

var Country = Countries.FirstOrDefault(o => o.Id == 100000581);
var personnelIds = Country.Personnels.Select(p => p.Id).ToArray();
personnelIds.Dump();

查询1 SQL

exec sp_executesql N'SELECT [t0].[Id], [t0].[Version], [t0].[Identifier], [t0].[Name], , [t0].[UpdatedBy] FROM [Personnel] AS [t0] WHERE [t0].[Country_Id] = @p0',N'@p0 bigint',@p0=100000581

查询2 LINQ

var Country = Countries.FirstOrDefault(o => o.Id == 100000581);
var personnelIds2 = Personnels.Where(p => p.Country == Country).Select(p => p.Id).ToArray();
personnelIds2.Dump();

查询2 SQL

exec sp_executesql N'SELECT [t0].[Id] FROM [Personnel] AS [t0] WHERE [t0].[Country_Id] = @p0',N'@p0 bigint',@p0=100000581

使用的数据库是SQL Express 2008.而LinqPad版本是4.43.06

Answer 1

//var Country = Countries.FirstOrDefault(o => o.Id == 100000581);
var personnelIds = context.Personnels
    .Where(p => p.Country.Id == 100000581)
    .Select(p => p.Id)
    .ToArray();

personnelIds.Dump();

试试这个，应该会更好。

Answer 2

人员集合将在访问时通过延迟加载填充，从而从数据库中检索所有字段。这是正在发生的事情......

// retrieves data and builds the single Country entity (if not null result)
var Country = Countries.FirstOrDefault(o => o.Id == 100000581);

// Country.Personnels accessor will lazy load and construct all Personnel entity objects related to this country entity object
// hence loading all of the fields
var personnelIds = Country.Personnels.Select(p => p.Id).ToArray();

你想要更像这样的东西：

// build base query projecting desired data
var personnelIdsQuery = dbContext.Countries
    .Where( c => c.Id == 100000581 )
    .Select( c => new
        {
            CountryId = c.Id,
            PersonnelIds = c.Personnels.Select( p => p.Id )
        }

// now do enumeration
// your example shows FirstOrDefault without OrderBy
// either use SingleOrDefault or specify an OrderBy prior to using FirstOrDefaul

var result = personnelIdsQuery.OrderBy( item => item.CountryId ).FirstOrDefault();

OR：

var result = personnelIdsQuery.SingleOrDefault();

然后获取ID数组（如果不为null）

if( null != result )
{
    var personnelIds = result.PersonnelIds;
}

Answer 3

尝试也可以尝试将人员分组到单个查询中

var groups =
    (from p in Personnel
     group p by p.CountryId into g
     select new 
     {
         CountryId = g.Key
         PersonnelIds = p.Select(x => x.Id)
     });
var personnelIds = groups.FirstOrDefault(g => g.Key == 100000581);

Answer 4

您是否在POCO for Personnel中明确定义了ForeignKey？将其保留在EF中是很常见的，但添加它会大大简化此代码和生成的SQL：

public class Personnel
{
    public Country Country { get; set; }

    [ForeignKey("Country")]
    public int CountryId { get; set; }

    . . .
}

> update-database -f -verbose

var ids = db.Personnel.Where(p => p.CountryId == 100000581).Select(p => p.Id).ToArray();

在LINQ中只选择一列

4 个答案: