重构以下LINQ查询以使其性能更好，看起来更简单

Question

Consider a function (GetData) which returns the following result set where the first column is division id and the second one is "TotalSales":

DivisionID: 3     500
DivisionID: 3     500
DivisionID: 3     500

DivisionID: 4     800     
DivisionID: 4     800

DivisionID: 5     50

I need to write a LINQ query to get the following result:

DiviosnID 3: (500 * 3) - 500 = 1000
DiviosnID 4: (800 * 2) - 800 = 800
DiviosnID 5: 0 /*this since it's happening only once*/

So the total becomes: 1000 + 800 = 1800

Finally, this value should be multiplied by -1, which results in -1800.

The following LINQ query gets the job done, however, it's IMSHO scary. The question is that whether it can be re-written to perform faster and look nicer?! Please note there's a third column there, just like the TotalSales named "TotalPurchases" which I need to do the same calculation for.

GetData()
.Where(t => t.DivisionId != 0)
.GroupBy(t => t.DivisionId)
.Where(g => g.Count() > 1)
.Select(g => new MyEntity
{
    TotalSales = g.Sum(n => n.TotalSales) - (g.Sum(n => n.TotalSales) / g.Count()),
    TotalPurchases = g.Sum(n => n.TotalPurchases) - (g.Sum(n => n.TotalPurchases) / g.Count())
})
.Union(Enumerable.Repeat(new MyEntity(), 1))
.Aggregate((t1, t2) => new MyEntity
{
    TotalSales = -(t1.TotalSales + t2.TotalSales),
    TotalPurchases = -(t1.TotalPurchases + t2.TotalPurchases),
});

Thank you

Answer 1

A quick first attempt:

var consolidatedData = GetData()
        .GroupBy(t => t.DivisionId)
        .Where(g => g.Skip(1).Any(i => i.DivisionId != 0))
        .Select(g => new
        {
            TotalSales = -(g.Sum(n => n.TotalSales) - g.Average(n => n.TotalSales)),
            TotalPurchases = -(g.Sum(n => n.TotalPurchases) - g.Average(n => n.TotalPurchases))
        });

var overallSales = consolidatedData.Sum(i => i.TotalSales);
var overallPurchases = consolidatedData.Sum(i => i.TotalPurchases);

Using Skip, you avoid the potentially expensive Count() being run on every group - it just Skips one item, and sees if there's anything left.

By building the result into an IEnumerable of anonyomous objects, the code is simplified - you can then query it afterewards when you want the final sums. Note that Average is also used in place of Sum / Count.

The Aggregate is removed - you only compute the final sum at the end when you need it.

Answer 2

My proposition:

var result = data
    .Where(t => t.DivisionID != 0)
    .GroupBy(t => t.DivisionID)
    .Select(g => new MyEntity
    {
        TotalSales = g.Sum(n => n.TotalSales) - g.Average(n => n.TotalSales),
        TotalPurchases = g.Sum(n => n.TotalPurchases) - g.Average(n => n.TotalPurchases)
    })
    .Aggregate(new MyEntity(), (t1, t2) => new MyEntity
    {
        TotalSales = t1.TotalSales - t2.TotalSales,
        TotalPurchases = t1.TotalPurchases - t2.TotalPurchases,
    });

Checking for Where(g => g.Count() > 1) is not necessary, because SUM and AVG for 1 element group is equal, so select will return 0. I also removed Union(Enumerable.Repeat(new MyEntity(), 1)) and I added seed to aggregate call - this is start value.

Another one:

var result = data
    .Where(t => t.DivisionID != 0)
    .GroupBy(t => t.DivisionID)
    .Select(g => new MyEntity
    {
        TotalSales = g.Sum(n => n.TotalSales) - g.Average(n => n.TotalSales),
        TotalPurchases = g.Sum(n => n.TotalPurchases) - g.Average(n => n.TotalPurchases)
    })
    .GroupBy(t => 0) // create single group
    .Select(g => new MyEntity
    {
        TotalSales = -g.Sum(t => t.TotalSales),
        TotalPurchases = -g.Sum(t => t.TotalPurchases)
    })
    .SingleOrDefault();

Assuming that TotalSales is constant for DivisionId you can use that:

var result = data
    .Where(t => t.DivisionID != 0)
    .GroupBy(t => t.DivisionID)
    .Select(g => new MyEntity
    {
        TotalSales = g.Skip(1).Sum(n => n.TotalSales),
        TotalPurchases = g.Skip(1).Sum(n => n.TotalPurchases)
    })
    .Aggregate(new MyEntity(), (t1, t2) => new MyEntity
    {
        TotalSales = t1.TotalSales - t2.TotalSales,
        TotalPurchases = t1.TotalPurchases - t2.TotalPurchases,
    });

var result = data
    .Where(t => t.DivisionID != 0)
    .GroupBy(t => t.DivisionID)
    .Select(g => new MyEntity
    {
        TotalSales = g.Skip(1).Sum(n => n.TotalSales),
        TotalPurchases = g.Skip(1).Sum(n => n.TotalPurchases)
    })
    .GroupBy(t => 0) // create single group
    .Select(g => new MyEntity
    {
        TotalSales = -g.Sum(t => t.TotalSales),
        TotalPurchases = -g.Sum(t => t.TotalPurchases)
    })
    .SingleOrDefault();