Fix to #35239 - EF9: SaveChanges() is significantly slower in .NET9 vs. .NET8 when using .ToJson() Mapping vs. PostgreSQL Legacy POCO mapping

[maumar]

Problem was that as part of AOT refactoring we changed way that we build comparers. Specifically, comparers of collections - ListOfValueTypesComparer, ListOfNullableValueTypesComparer and ListOfReferenceTypesComparer. Before those list comparer Compare, Hashcode and Snapshot methods would take as argument element comparer, which was responsible for comparing elements. We need to be able to express these in code for AOT but we are not able to generate constant of type ValueComparer (or ValueComparer) that was needed. As a solution, each comparer now stores expression describing how it can be constructed, so we use that instead (as we are perfectly capable to expressing that in code form). Problem is that now every time compare, snapshot or hashcode method is called for array type, we construct new ValueComparer for the element type. As a result in the reported case we would generate 1000s of comparers which all have to be compiled and that causes huge overhead.

Fix is to pass relevant func from the element comparer to the outer comparer. We only passed the element comparer object to the outer Compare/Hashcode/Snapshot function to call that relevant func. This way we avoid constructing redundant comparers.

Fixes #35239

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[maumar]

note: this PR only addresses ListOfValueTypesComparer and ListOfNullableValueTypesComparer - the issue also exists for ListOfReferenceTypesComparer and in Cosmos, but those are more tricky to fix as the TElement of the parent comparer doesn't necessarily match the TElement of the element comparer, so it's not always safe to copy over lambdas from element to parent.
THe aim is to make this PR safe enough for potential servicing and as it fixes the reported case (List<Guid>)

[roji]

@maumar for reference, could you include the benchmark results before and after this change, as well as the benchmark code itself?

[maumar]

I've updated the pr with cleaner approach. We no longer need the legacy code path, instead we reason about compatibility between lambda signatures (expected in the list comparer method and actual that we get from element comparer). I'm ok with going with the converting comparer as well, which works across the board, is more elegant and terse solution for sure. Pros of the solution here is no new APIs and a bit simpler Equals/Hashcode/Snapshot code - we don't always need to convert, e.g. when case of list of arrays (List<int[]>), outer comparer expects Equals of signature Func<int[], int[], bool> but element selector is typed as ValueComparer<IEnumerable>. However in this case we can get away with just passing the Func<IE<int>, IE<int>, bool> into Func<int[], int[], bool>. Problem only appears for snapshot (can't fit Func<IE<int>, IE<int>> into Func<int[], int[]>) so we do the rewite only there. ended up incorporating @roji 's concept of ConvertingValueComparer, but took advantage of contravariance - only doing conditional conversion based on assignability of target comparer type from the source type..

[maumar]

perf numbers

with warmup, so that comparers are/should be compiled:
8.0 - 77ms
9.0 - 593ms
9.0.2 - 98ms

no warmup:
8.0 - 84ms
9.0 - 650 ms
9.0.2 - 132ms

Will convert it to proper BDN and post code and more accurate numbers. But the improvement is significant.

[maumar]

BDN numbers:

8.0.11

Method	Mean	Error	StdDev
SaveChangesTest	172.1 ms	1.78 ms	1.58 ms

9.0

Method	Mean	Error	StdDev
SaveChangesTest	5.487 s	0.0621 s	0.0551 s

9.0.2 (with fix)

Method	Mean	Error	StdDev
SaveChangesTest	179.8 ms	1.71 ms	1.43 ms

benchmark code:

// See https://aka.ms/new-console-template for more information
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Running;
using Microsoft.EntityFrameworkCore;

BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args, DefaultConfig.Instance);

public class SaveChangesBenchmark
{
    private MyContext _ctx = new MyContext();

    [GlobalSetup]
    public virtual async Task Initialize()
    {
        var ctx = new MyContext();
        await ctx.Database.EnsureDeletedAsync();
        await ctx.Database.EnsureCreatedAsync();

        var seedData = ReturnSeedData();
        ctx.SampleEntities.AddRange(seedData);
        ctx.SaveChanges();
    }

    [IterationSetup]
    public void Setup()
    {
        _ctx = new MyContext();
        _ctx.ChangeTracker.Clear();

        foreach (var testId in Enumerable.Range(1, 200))
        {
            var src = _ctx.SampleEntities.Where(a => a.TestId == testId).FirstOrDefault();
        }
    }

    [Benchmark]
    public async Task SaveChangesTest()
    {
        await _ctx.SaveChangesAsync();
    }

    private static List<SampleEntity> ReturnSeedData()
    {
        var list = new List<SampleEntity>();

        for (int i = 0; i < 200; i++)
        {
            var s = new SampleEntity
            {
                TestId = i,
                RockId = Guid.NewGuid(),
                SccId = Guid.NewGuid(),
                AnotherId = Guid.NewGuid(),
                IsPushed = false,
                SeId = Guid.NewGuid(),
                SampleId = Guid.NewGuid(),
                Jsons = []
            };

            for (int j = 0; j < 300; j++)
            {
                var studentGradingNeed = new SampleJson
                {
                    ComponentGroupId = Guid.NewGuid(),
                    ComponentId = Guid.NewGuid(),
                    MonthId = Guid.NewGuid(),
                    IsGroupLevel = false,
                    MeasureId = Guid.NewGuid(),
                    OrderedComponentTrackingId = Guid.NewGuid(),
                    Result = new ResultJson
                    {
                        CreatedBy = Guid.NewGuid(),
                        CreatedByName = "Test",
                        CreatedDate = DateTime.UtcNow,
                        LastModifiedBy = Guid.NewGuid(),
                        LastModifiedByName = "Test",
                        LastModifiedDate = DateTime.UtcNow,
                        MarkIdValue = Guid.NewGuid(),
                        NumericValue = 44,
                        CommentIds = new List<Guid> { Guid.NewGuid() },
                        TextValue = "FF"
                    }
                };

                s.Jsons.Add(studentGradingNeed);
            }

            list.Add(s);
        }

        return list;
    }
}

public class MyContext : DbContext
{
    public DbSet<SampleEntity> SampleEntities { get; set; } = null!;

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=ReproSaveChangesBDN;Trusted_Connection=True;MultipleActiveResultSets=true");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<SampleEntity>().OwnsMany(c => c.Jsons, d =>
        {
            d.ToJson();
            d.OwnsOne(e => e.Result);
        });
    }
}


public class SampleEntity
{
    public Guid Id { get; set; }
    public Guid SampleId { get; set; }
    public Guid AnotherId { get; set; }
    public int TestId { get; set; }
    public Guid RockId { get; set; }
    public Guid SccId { get; set; }
    public Guid SeId { get; set; }

    public List<SampleJson> Jsons { get; set; } = [];
    public bool IsPushed { get; set; }
}

public record SampleJson
{

    public Guid TrackingId { get; set; }
    public Guid ComponentGroupId { get; set; }
    public Guid ComponentId { get; set; }
    public Guid? OrderedComponentTrackingId { get; set; }
    public Guid? MonthId { get; set; }
    public Guid? MeasureId { get; set; }
    public bool IsGroupLevel { get; set; }
    public ResultJson? Result { get; set; }
}

public record ResultJson
{
    public string? TextValue { get; set; }
    public decimal? NumericValue { get; set; }
    public Guid? MarkIdValue { get; set; }
    public List<Guid> CommentIds { get; set; } = [];
    public Guid CreatedBy { get; set; }
    public string CreatedByName { get; set; } = null!;
    public DateTime CreatedDate { get; set; }
    public Guid LastModifiedBy { get; set; }
    public string LastModifiedByName { get; set; } = null!;
    public DateTime LastModifiedDate { get; set; }
}

public interface ISampleDbContext
{
    DbSet<SampleEntity> SampleEntities { get; set; }
    void SetConnectionString(string connectionString);
}

[maumar]

I removed the constraint as conversions happen both ways

in case of object[] { 1, 2, 3 } target is target is object and source is int
in case of nested lists (List<List> target is List and source is object (because element comparer is ListOfReferenceTypesComparer which is typed as ValueComparer