[英]Performing two queries in a single round trip to the database
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,但它需要两次往返数据库。 为了优化代码,有没有办法重新编写这个查询,所以它只有一次往返数据库?
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
将看起来那样:
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
我不确定它是否足够值,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)在应用寻呼参数后实现查询。
诀窍是使用由EF支持的常量值group by
。 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>();
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