实体框架 GroupBy 采用 mySQL 最古老的

我有一个巨大的项目列表，需要按一个属性对它们进行分组。然后应该选择每组中最古老的。

简化示例：选择每个用户中最旧的用户FirstName.

using (ED.NWEntities ctx = new ED.NWEntities())
{
    IQueryable<ED.User> Result = ctx.User.GroupBy(x => x.FirstName)
                                    .Select(y => y.OrderBy(z => z.BirthDate)
                                    .FirstOrDefault())
                                    .AsQueryable();
}

Class User:

public partial class User
{
    public int UserID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public Nullable<System.DateTime> BirthDate { get; set; }
}

我想知道为什么这个语句花了这么长时间才设置断点Result并查看生成的 SQL 语句：

{SELECT
`Apply1`.`UserID`, 
`Apply1`.`FIRSTNAME1` AS `FirstName`, 
`Apply1`.`LastName`, 
`Apply1`.`BirthDate`
FROM (SELECT
`Distinct1`.`FirstName`, 
(SELECT
`Project2`.`UserID`
FROM `User` AS `Project2`
 WHERE (`Distinct1`.`FirstName` = `Project2`.`FirstName`) OR ((`Distinct1`.`FirstName` IS  NULL) AND (`Project2`.`FirstName` IS  NULL))
 ORDER BY 
`Project2`.`BirthDate` ASC LIMIT 1) AS `UserID`, 
(SELECT
`Project2`.`FirstName`
FROM `User` AS `Project2`
 WHERE (`Distinct1`.`FirstName` = `Project2`.`FirstName`) OR ((`Distinct1`.`FirstName` IS  NULL) AND (`Project2`.`FirstName` IS  NULL))
 ORDER BY 
`Project2`.`BirthDate` ASC LIMIT 1) AS `FIRSTNAME1`, 
(SELECT
`Project2`.`LastName`
FROM `User` AS `Project2`
 WHERE (`Distinct1`.`FirstName` = `Project2`.`FirstName`) OR ((`Distinct1`.`FirstName` IS  NULL) AND (`Project2`.`FirstName` IS  NULL))
 ORDER BY 
`Project2`.`BirthDate` ASC LIMIT 1) AS `LastName`, 
(SELECT
`Project2`.`BirthDate`
FROM `User` AS `Project2`
 WHERE (`Distinct1`.`FirstName` = `Project2`.`FirstName`) OR ((`Distinct1`.`FirstName` IS  NULL) AND (`Project2`.`FirstName` IS  NULL))
 ORDER BY 
`Project2`.`BirthDate` ASC LIMIT 1) AS `BirthDate`
FROM (SELECT DISTINCT 
`Extent1`.`FirstName`
FROM `User` AS `Extent1`) AS `Distinct1`) AS `Apply1`}

问：有没有办法更高效的解决他？子选择的成本很高，并且 EF 为每一列生成一个子选择。我使用 mySQL .NET 连接器版本 6.9.5.0

使用乔恩·斯基特的answer https://stackoverflow.com/questions/489258/linqs-distinct-on-a-particular-property在不同的..

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
    (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    HashSet<TKey> seenKeys = new HashSet<TKey>();
    foreach (TSource element in source)
    {
        if (seenKeys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
}

你可以试试：

using (ED.NWEntities ctx = new ED.NWEntities())
{
    IQueryable<ED.User> Result = ctx.User.OrderBy(y => y.BirthDate)
                                    .DistinctBy(z => z.FirstName)
                                    .AsQueryable();
}