Question

我在下面有代码示例查询产品列表。

 var productResults = Products.Where((p) => refFilterSequence.Contains(p.Ref))
                .GroupBy(g => g.Code, (key, g) => g.OrderBy(whp => whp.Ref).First()).ToList();

这完全符合预期，并在使用内存集合时返回我想要的4行，但是在针对Oracle数据库运行时：

.GroupBy(g => g.Code, (key, g) => g.OrderBy(whp => whp.Ref).First())

这会抛出一个错误，说我应该使用Oracle数据库不支持的FirstOrDefault。错误 oracle 11.2.0.3.0不支持应用被抛出。谷歌在CodePlex上展示了这一点：https://entityframework.codeplex.com/workitem/910。

使用以下二进制文件时会发生这种情况：

EntityFramework 6.0.0.0
Oracle.ManagedDataAccess 4.121.2.0
Oracle.ManagedDataAccess.EntityFramework 6.121.2.0
.Net Framework 4.5.1

该数据库是Oracle 11.2.0.3.0数据库。

生成的sql使用OUTER APPLY（见下图），Oracle 11.2.0.3.0版本不支持，那么为什么EF / Oracle.ManagedDataAccess会尝试使用它？有没有办法告诉EF不要使用APPLY关键字？

SQL

下面的页面显示在Oracle 12c第1版中添加了APPLY支持，但我无法更新所有数据库只是为了使GROUP BY工作。 http://www.oracle.com/technetwork/database/windows/newfeatures-084113.html

这似乎是一个已知问题（Known Issues in SqlClient for Entity Framework）：

以下是一些可能导致这种情况的典型情况   输出中存在CROSS APPLY和/或OUTER APPLY运算符   查询：


使用接受元素选择器的分组方法的LINQ查询。

在我开始创建视图之前（我必须在几个数据库上创建视图），任何人都可以看到另一个解决方案吗？

对于任何感兴趣的人来说，针对这个数据库版本执行我想做的SQL看起来如下所示：

select *
from ( select  RANK() OVER (PARTITION BY sm.product ORDER BY refs.map)      ranking, sm.*
            from    schema.table sm,
                    (
                        select 'R9' ref, 0 map from dual
                        union all
                        select 'R1' ref, 1 map from dual
                        union all
                        select 'R6' ref, 2 map from dual
                    ) refs
            where   sm.ref= refs.ref                               
          ) stock
where ranking  = 1

代码最终将在传递给Web API中的OData控制器的服务类中。以下示例使用演示数据，真实数据库有700,000条记录，所以我想避免执行查询，让OData处理页面限制和进一步过滤。

using System;
using System.Collections.Generic;
using System.Linq;

namespace DemoApp
{
    class Program
    {
        public class Product
        {
            public string Ref { get; set; }
            public string Code { get; set; }
            public int Quantity { get; set; }
        }

        //demo data
        static readonly List<Product> Products = new List<Product>
        {
            new Product { Ref = "B2", Code = "ITEM1", Quantity = 1},
            new Product { Ref = "R1", Code = "ITEM1", Quantity = 2},
            new Product { Ref = "R9", Code = "ITEM1", Quantity = 3},
            new Product { Ref = "R9", Code = "ITEM2", Quantity = 4},
            new Product { Ref = "R6", Code = "ITEM2", Quantity = 5},
            new Product { Ref = "B2", Code = "ITEM3", Quantity = 6},
            new Product { Ref = "R1", Code = "ITEM3", Quantity = 7},
            new Product { Ref = "R9", Code = "ITEM3", Quantity = 8},
            new Product { Ref = "B2", Code = "ITEM4", Quantity = 9},
            new Product { Ref = "X3", Code = "ITEM4", Quantity = 10},
            new Product { Ref = "B8", Code = "ITEM5", Quantity = 10},
            new Product { Ref = "R6", Code = "ITEM5", Quantity = 12},
            new Product { Ref = "M2", Code = "ITEM5", Quantity = 13},
            new Product { Ref = "R1", Code = "ITEM5", Quantity = 14},
        };

    static void Main(string[] args)
    {
        // this array is of variable length, and will not always contain 3 items.
        var refFilterSequence = new List<string> {"R9", "R1", "R6"};

        var results = GetProductsForODataProcessing(refFilterSequence);

        // some further filtering may occur after the queryable is returned.
        // the actual implmentation is an OData Web API, so filters, expansions etc could be added.

        //results = results.Where(p => p.Quantity > 2);

        results.ToList().ForEach(p => Console.WriteLine("RANK:{0}\tREF:{1}\tCode:{2}\tQty:{3}", "?", p.Ref, p.Code, p.Quantity));
        Console.ReadLine();
    }

    static IQueryable<Product> GetProductsForODataProcessing(List<string> filterSequence )
    {
        var productResults = Products.Where((p) => filterSequence.Contains(p.Ref))
            .GroupBy(g => g.Code, (key, g) => g.OrderBy(whp => whp.Ref).First()).AsQueryable();

        return productResults;               
    }
}


// Example Output
// .......................
// REF:R1 Code:ITEM1 Qty:2
// REF:R6 Code:ITEM2 Qty:3
// REF:R1 Code:ITEM3 Qty:7
// REF:R1 Code:ITEM5 Qty:14

Answer 1

因为您可以自己编写查询。也许您可以使用它创建存储过程并从实体框架中调用SP。

Answer 2

您可以尝试在GroupBy之前执行ToArray，因此它会在内存中执行。它不是最佳性能，但应该有效。

 var productResults = Products.Where((p) => refFilterSequence.Contains(p.Ref)).ToArray()
                    .GroupBy(g => g.Code, (key, g) => g.OrderBy(whp => whp.Ref).First()).ToList();

Answer 3

Oracle 11不支持APPLY。但是，Oracle 12确实如此。

Linq to Entities Group By（OUTER APPLY）＆＃34; oracle 11.2.0.3.0不支持apply＆＃34;

3 个答案: