为什么 EF Core 最后要添加一个额外的 ORDER BY

Question

我有一个问题：

    public async Task<IEnumerable<CategoryDto>> GetCategoriesAsync()
    {
        var dto = await _context.Categories.FromSqlRaw(
                @"SELECT * FROM [cp].[GetCategoryTree]")
            .ProjectTo<CategoryDto>(_mapper.ConfigurationProvider)
            .AsNoTrackingWithIdentityResolution()
            .ToListAsync();

        return dto;
    }

它产生这个 sql：

SELECT [c].[banner_group_id], [c].[category_id], [c].[description], [c].[inactive], [c].[issitecategory], [c].[keywords], [c].[category_name], [c].[page_content], [c].[parent_category_id], [c].[sort_order], [c].[title], [b].[banner_group_id], [b].[category_id], [b].[description], [b].[inactive], [b].[issitecategory], [b].[keywords], [b].[category_name], [b].[page_content], [b].[parent_category_id], [b].[sort_order], [b].[title]

FROM (

    SELECT * FROM [cp].[GetCategoryTree]

) AS [c]

LEFT JOIN [bma_ec_categories] AS [b] ON [c].[category_id] = [b].[parent_category_id]

ORDER BY [c].[category_id], [b].[category_id]

[cp].[GetCategoryTree] 的输出已在该视图中排序。为什么 EF Core 最后要添加一个额外的ORDER BY？我可以告诉 EF Core 不排序吗？

Answer 1

递归CTE返回结果是普通的记录集，也就是说如果你在客户端需要树，你必须自己重构它。

让我们重用原始查询：

public async Task<IEnumerable<CategoryDto>> GetCategoriesAsync()
{
    var plainResult = await _context.Categories.FromSqlRaw(@"SELECT * FROM [cp].[GetCategoryTree]")
        .Select(c => new CategoryDto
        {
            CategoryId = c.CategoryId,
            ParentId = c.ParentId,
            Name = c.Name,
            SortOrder = c.SortOrder
        })
        .ToListAsync();

       var lookup = plainResult.Where(x => x.ParentId != 0).ToLookup(x => x.ParentId);

       foreach (c in plainResult)
       {
           if (lookup.ContainsKey(c.CategoryId))
           {
               c.Children = lookup[c.CategoryId]
                   .OrderBy(x => x.SortOrder)
                   .ThenBy(x => x.Name)
                   .ToList();
           }            
       }

    var dto = plainResult.Where(x => x.ParentId == 0).ToList();

    return dto;
}

Answer 2

我从一个平面 Category 查询开始。我需要它在树结构中。所以我为Children 添加了一个Include。但它只包括根元素上的子元素。我问了 SO here 并被指向 Recursive CTE。这就是[cp].[GetCategoryTree]。我现在已经放弃了所有这些并提出了我的问题：

    public async Task<IEnumerable<CategoryDto>> GetCategoriesAsync()
    {
       var dto = await _context.Categories
            .Where(x => x.ParentId == 0)
            .OrderBy(x=> x.SortOrder)
            .ThenBy(x => x.Name)
            .ProjectTo<CategoryDto>(_mapper.ConfigurationProvider)
            .AsNoTracking()
            .ToListAsync();

       foreach (var categoryDto in dto)
       {
           foreach (var categoryDtoChild in categoryDto.Children)
           {
               categoryDtoChild.Children = await _context.Categories
                   .Where(x => x.ParentId == categoryDtoChild.CategoryId)
                   .OrderBy(x => x.SortOrder)
                   .ThenBy(x => x.Name)
                   .ProjectTo<CategoryDto>(_mapper.ConfigurationProvider)
                   .AsNoTracking()
                   .ToListAsync();
            }
       }

        return dto;
    }

这很有效，慢慢确定，但一直填充到曾孙。如果有人有更好的解决方案，我会全力以赴。

基准测试结果（我的和@SvyatoslavDanyliv）：

Method	IterationCount	Mean	Error	StdDev	Median	Min	Max	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
GetCategories	1	142.82 ms	5.260 ms	14.921 ms	142.65 ms	85.572 ms	170.29 ms	1.00	0.00	1000.0000	-	-	5,845 KB
GetCategoriesSingleQuery	1	11.17 ms	0.501 ms	1.396 ms	10.87 ms	8.057 ms	14.76 ms	0.08	0.01	-	-	-	914 KB
GetCategories	2	290.03 ms	7.703 ms	21.473 ms	288.38 ms	239.879 ms	350.48 ms	1.00	0.00	2000.0000	-	-	11,637 KB
GetCategoriesSingleQuery	2	23.21 ms	1.815 ms	5.207 ms	21.58 ms	17.005 ms	38.73 ms	0.08	0.02	-	-	-	1,745 KB

我想我会接受@SvyatoslavDanyliv 的回答。