在一次数据库往返中执行两个查询

Question

我有以下代码来执行全文搜索。它创建一个查询，获取该查询返回的总行数，然后仅检索当前页面的实际行数。

// Create IQueryable
var query = from a in ArticleServerContext.Set<Article>()
            where a.Approved
            orderby a.UtcDate descending
            select a;

// Get total rows (needed for pagination logic)
int totalRows = query.Count()

// Get rows for current page
query = query.Skip((CurrentPage - 1) * RowsPerPage).Take(RowsPerPage);

这工作正常，但需要两次往返数据库。为了优化代码，是否有任何方法可以重新处理此查询，使其只往返数据库一次？

Answer 1

是的，您可以在对数据库的唯一一个查询的帮助下执行这两个操作：

// Create IQueryable
var query = from a in ArticleServerContext.Set<Article>()
            where a.Approved
            orderby a.UtcDate descending
            select new { a, Total = ArticleServerContext.Set<Article>().Where(x => x.Approved).Count() };

//Get raw rows for current page with Total(Count) field
var result = query.Skip((CurrentPage - 1) * RowsPerPage).Take(RowsPerPage).ToList();

//this data you actually will use with your logic 
var actualData = result.Select(x => x.a).ToList();

// Get total rows (needed for pagination logic)
int totalRows = result.First().Total;

如果您使用 MSSQL query 将是这样的：

SELECT 
    [Extent1].[ID] AS [ID], 
    [Extent1].[UtcDate] AS [UtcDate], 
    [Extent1].[Approved] AS [Approved],     
    [GroupBy1].[A1] AS [C1]
    FROM  [dbo].[Articles] AS [Extent1]
    CROSS JOIN  (SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Articles] AS [Extent2]
        WHERE [Extent2].[Approved] ) AS [GroupBy1]
    WHERE [Extent1].[Approved]
    ORDER BY [Extent1].[UtcDate] DESC

Answer 2

我不确定它是否足够值得，但在以下限制条件下它是可行的：

(1) CurrentPage 和 RowsPerPage 不受 totalRows 值的影响。
(2) 查询在应用分页参数后具体化。

诀窍是使用 group by 常量值，这是 EF 支持的。代码如下所示：

var query = 
    from a in ArticleServerContext.Set<Article>()
    where a.Approved
    // NOTE: order by goes below
    group a by 1 into allRows
    select new
    {
        TotalRows = allRows.Count(),
        PageRows = allRows
            .OrderByDescending(a => a.UtcDate)
            .Skip((CurrentPage - 1) * RowsPerPage).Take(RowsPerPage)
    };

var result = query.FirstOrDefault();
var totalRows = result != null ? result.TotalRows : 0;
var pageRows = result != null ? result.PageRows : Enumerable.Empty<Article>();

在一次数据库往返中执行两个查询

Performing two queries in a single round trip to the database

c#

sql

linq

iqueryable