EntityFramework LINQ 查询计数失败但查询 returns 结果。如何优化 LINQ 查询？

Question

我有以下执行自左外连接的 LINQ 查询。查询看起来有点复杂，但只是对其自身进行自连接（目的是将每条记录与前一个工作日的记录连接起来），然后进行一些参数化过滤。

var newBreakThreshold = decimal.Parse(WebConfigurationManager.AppSettings["NewBreakThreshold"]);
using (var dbContext = new NavFoToBoCompareDbContext())
{
    var query = from current in dbContext.NAVSummaries
                let currentWD = SqlFunctions.DatePart("dw", current.ValueDate)
                let currentPD = DbFunctions.AddDays(current.ValueDate, currentWD == 2 ? -3 : currentWD == 1 ? -2 : -1).Value
                join previous in dbContext.NAVSummaries
                on new { current.Portfolio, PD = currentPD }
                equals new { previous.Portfolio, PD = previous.ValueDate }
                into previousGroup
                from previous in previousGroup.DefaultIfEmpty() // LEFT OUTER JOIN
                select new { outer = current, inner = previous };

    if (dateStart.HasValue)
        query = query.Where(e => e.outer.ValueDate >= dateStart.Value);
    if (dateEnd.HasValue)
        query = query.Where(e => e.outer.ValueDate <= dateEnd.Value);
    if (!portfolio.Equals("ALL", StringComparison.OrdinalIgnoreCase))
        query = query.Where(e => e.outer.Portfolio.Equals(portfolio, StringComparison.OrdinalIgnoreCase));
    if (!owner.Equals("ALL", StringComparison.OrdinalIgnoreCase))
        query = query.Where(e => e.outer.PortfolioOwner.Equals(owner, StringComparison.OrdinalIgnoreCase));
    if (status != 0)
        query = query.Where(e => e.outer.Statuses.Any(s => s.StatusId == status));

    var query2 = query.Select(s => new
                {
                    BackOfficeNAV = s.outer.BackOfficeNAV,
                    FrontOfficeNAV = s.outer.FrontOfficeNAV,
                    Threshold = s.outer.Threshold,
                    ExtractId = s.outer.ExtractId,
                    ExtractStatus = s.outer.ExtractStatus,
                    PortfolioOwner = s.outer.PortfolioOwner,
                    DateTimeModified = s.outer.DateTimeModified,
                    MostCorrectNAV = s.outer.MostCorrectNAV,
                    Comments = s.outer.Comments,
                    Statuses = s.outer.Statuses,
                    Extracts = s.outer.Extracts,
                    Portfolio = s.outer.Portfolio,
                    ValueDate = s.outer.ValueDate,
                    DifferencePercent = s.outer.DifferencePercent,
                    DayOverDayChange = s.outer.DifferencePercent - s.inner.DifferencePercent,
                    IsChange = s.inner.DifferencePercent != s.outer.DifferencePercent,

                    PreviousValueDate = s.inner.ValueDate,
                    PreviousDifferencePercent = s.inner.DifferencePercent
                });


    query2 = query2.Where(r => "NEW".Equals(breakOption, StringComparison.OrdinalIgnoreCase) ?
                                                ((r.DifferencePercent > r.Threshold) && r.IsChange && r.DayOverDayChange > newBreakThreshold) :
                                "OLD".Equals(breakOption, StringComparison.OrdinalIgnoreCase) ? (r.DifferencePercent > r.Threshold) :
                                "ALL".Equals(breakOption, StringComparison.OrdinalIgnoreCase));

    var resultCount = query2.Count();
}

查询用在两个地方。在一种方法中，它用于计算分页所需的计数。在另一种方法中，它用于从数据库中获取实际结果。获取更大结果集的实际结果的实现成功执行，而 Count() 查询失败并出现超时异常。 Note:Both 实现完全相同。

有人可以帮我优化这个查询吗？提前致谢。

Answer 1

您可以简单地使用 AsParallel() 启用 linq 查询的多线程执行。然后您应该检查表的索引以提高性能。

Answer 2

不太确定这是问题所在，但至少让我们尝试通过手动构建表达式消除 dateStart / dateEnd 参数来消除所谓的 Parameter Sniffing Problem 的潜在影响具有恒定值。

先来个小帮手方法：

using System;
using System.Linq;
using System.Linq.Expressions;

public static class QueryableUtils
{
    public static IQueryable<T> WhereBetween<T>(this IQueryable<T> source, Expression<Func<T, DateTime>> dateSelector, DateTime? startDate, DateTime? endDate)
    {
        if (startDate == null && endDate == null) return source;
        var startCond = startDate != null ? Expression.GreaterThanOrEqual(dateSelector.Body, Expression.Constant(startDate.Value)) : null;
        var endCond = endDate != null ? Expression.LessThanOrEqual(dateSelector.Body, Expression.Constant(endDate.Value)) : null;
        var predicate = Expression.Lambda<Func<T, bool>>(
            startCond == null ? endCond : endCond == null ? startCond : Expression.AndAlso(startCond, endCond),
            dateSelector.Parameters[0]);
        return source.Where(predicate);
    }
}

然后尝试以下方法，看看是否有帮助：

//if (dateStart.HasValue)
//    query = query.Where(e => e.outer.ValueDate >= dateStart.Value);
//if (dateEnd.HasValue)
//    query = query.Where(e => e.outer.ValueDate <= dateEnd.Value);
query = query.WhereBetween(e => e.outer.ValueDate, dateStart, dateEnd);

EntityFramework LINQ 查询计数失败但查询 returns 结果。如何优化 LINQ 查询？

EntityFramework LINQ query count fails but query returns result. How to optimize LINQ query?

c#

linq

sql-server

linq-to-entities

entity-framework