Why DefaultIfEmpty inside double SelectMany is wrong working? #33343

KvaKoZyaBBrr · 2024-03-18T08:59:08Z

I have entities:

class MyRecord<T>
{
    public T Item {get;set;}
}
class JoinResult<T1, T2>
{
    public T1 Left { get; set; }
    public T2 Right { get; set; }
}
class  TestItem
{
    public int EntityId {get;set;}
    public string Name {get;set;}
    public int ParentId {get;set;} // link to TestItem.EntityId
}
class  TestQuery
{
    public int QueryId {get;set;}
}

And when Im try get all childs for requested items ordered by desc which has linked query I use next linq:

DbSet<MyRecord<long>>().FromSql(select * from unnest({0}), object[] { NpgsqlParameter<MyRecord<long>[]> })
.SelectMany(pk => dataContext.TestItems
    .Where(item => pk.Item == item.ParentId)
    .SelectMany(
        collectionSelector: entity => dataContext.TestQueries
            .Where(a => a.QueryId == entity.EntityId)
            .DefaultIfEmpty(), 
        resultSelector: (left, right) => new JoinResult<TestItem, TestQuery>{ 
            Left = left, 
            Right = right 
        }
    )
    .OrderByDescending(result => result.Right.ObjectId)
    .Select(result => result.Left))

I expect DefaultIfEmpty change inner SelectMany to leftJoin inside join by outer SelectMany smth

SELECT t0.EntityId, t0.Name, t0.ParentId
FROM (
    select * from unnest(ARRAY[101,102])
) AS e
JOIN LATERAL (
    SELECT c.EntityId, c.Name, c.ParentId
    FROM items AS c
    LEFT JOIN LATERAL (
        SELECT s.QueryId
        FROM queries AS q
        WHERE q.QueryId = c.EntityId
    ) AS t ON TRUE
    WHERE e.unnest = c.ParentId
    ORDER BY t.QueryId DESC
) AS t0 ON TRUE

But I get

SELECT t0.EntityId, t0.Name, t0.ParentId
FROM (
    select * from unnest(ARRAY[101,102])
) AS e
LEFT JOIN LATERAL (
    SELECT c.EntityId, c.Name, c.ParentId
    FROM items AS c
    JOIN LATERAL (
        SELECT s.QueryId
        FROM queries AS q
        WHERE q.QueryId = c.EntityId
    ) AS t ON TRUE
    WHERE e.unnest = c.ParentId
    ORDER BY t.QueryId DESC
) AS t0 ON TRUE

How can I change this behaviour?

roji · 2024-03-22T15:33:50Z

@KvaKoZyaBBrr can you please submit a runnable sample, as a console program I can actually run and see the results? Your query code there doesn't seem to compile.

KvaKoZyaBBrr · 2024-03-22T17:10:54Z

Of course.
TestEF_selectMany.zip
Im work on postgres so assert write on postgreSQL. But in SQL I expect same behaviour in query

roji · 2024-03-23T09:42:51Z

Thanks for the repro. I've removed the irrelevant parts of the query (that's always a good idea), here's a distlilled comparison of with and without DefaultIsEmpty for better understanding what's going on:

var withDefaultIfEmpty = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Id)
        .SelectMany(
            entity => context.TestEntities
                .Where(a => a.Name.Contains(entity.Name))
                .DefaultIfEmpty()));
Console.WriteLine(withDefaultIfEmpty.ToQueryString());

var withoutDefaultIfEmpty = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Id)
        .SelectMany(
            entity => context.TestEntities
                .Where(a => a.Name.Contains(entity.Name))));
Console.WriteLine(withoutDefaultIfEmpty.ToQueryString());

SQLs:

SELECT [s].[Id], [s].[Name], [s].[Parent]
FROM [TestEntities] AS [t]
LEFT JOIN (
    SELECT [t2].[Id], [t2].[Name], [t2].[Parent], [t0].[Id] AS [Id0]
    FROM [TestEntities] AS [t0]
    CROSS APPLY (
        SELECT [t1].[Id], [t1].[Name], [t1].[Parent]
        FROM [TestEntities] AS [t1]
        WHERE [t1].[Name] IS NOT NULL AND [t0].[Name] IS NOT NULL AND (CHARINDEX([t0].[Name], [t1].[Name]) > 0 OR [t0].[Name] LIKE N'')
    ) AS [t2]
) AS [s] ON [t].[Id] = [s].[Id0]

SELECT [s].[Id], [s].[Name], [s].[Parent]
FROM [TestEntities] AS [t]
INNER JOIN (
    SELECT [t2].[Id], [t2].[Name], [t2].[Parent], [t0].[Id] AS [Id0]
    FROM [TestEntities] AS [t0]
    CROSS APPLY (
        SELECT [t1].[Id], [t1].[Name], [t1].[Parent]
        FROM [TestEntities] AS [t1]
        WHERE [t1].[Name] IS NOT NULL AND [t0].[Name] IS NOT NULL AND (CHARINDEX([t0].[Name], [t1].[Name]) > 0 OR [t0].[Name] LIKE N'')
    ) AS [t2]
) AS [s] ON [t].[Id] = [s].[Id0]

First, regarding why the outer join is a LEFT JOIN... The fragment inside the outer SelectMany() ends with a DefaultIsEmpty(); this means that for every outer ID, if there's no matching TestEntity, the query should return a null. If the outer query used a non-LEFT INNER JOIN, it would instead filter out any IDs with non-matching TestEntities, which would return wrong result (remember, EF always tries to produce the same results that a regular LINQ-to-Object query would produce).

But more generally, your usage of SelectMany() is slightly odd, and likely inefficient... Taking your original fragmentary query above:

DbSet<MyRecord<long>>().FromSql(select * from unnest({0}), object[] { NpgsqlParameter<MyRecord<long>[]> })
.SelectMany(pk => dataContext.TestItems
    .Where(item => pk.Item == item.ParentId)

What you're looking for here seems to be a simple Contains query:

var ids = new[] { 1, 2, 3 };
_ = context.TestEntities.Where(e => ids.Contains(e.Id))...

This should make the EF provider produce better SQL, using the PG ANY operator, rather than using custom SQL with unnest and SelectMany(), which translates to costly extra joins.

dombaigabor · 2024-03-25T12:22:55Z

We have the same issue with nested SelectMany (MSSQL, EF Core 7.0.5.).

Thanks for the repro. I've removed the irrelevant parts of the query (that's always a good idea), here's a distlilled comparison of with and without DefaultIsEmpty for better understanding what's going on:

var withDefaultIfEmpty = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Id)
        .SelectMany(
            entity => context.TestEntities
                .Where(a => a.Name.Contains(entity.Name))
                .DefaultIfEmpty()));
Console.WriteLine(withDefaultIfEmpty.ToQueryString());

var withoutDefaultIfEmpty = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Id)
        .SelectMany(
            entity => context.TestEntities
                .Where(a => a.Name.Contains(entity.Name))));
Console.WriteLine(withoutDefaultIfEmpty.ToQueryString());

SQLs:

SELECT [s].[Id], [s].[Name], [s].[Parent]
FROM [TestEntities] AS [t]
LEFT JOIN (
    SELECT [t2].[Id], [t2].[Name], [t2].[Parent], [t0].[Id] AS [Id0]
    FROM [TestEntities] AS [t0]
    CROSS APPLY (
        SELECT [t1].[Id], [t1].[Name], [t1].[Parent]
        FROM [TestEntities] AS [t1]
        WHERE [t1].[Name] IS NOT NULL AND [t0].[Name] IS NOT NULL AND (CHARINDEX([t0].[Name], [t1].[Name]) > 0 OR [t0].[Name] LIKE N'')
    ) AS [t2]
) AS [s] ON [t].[Id] = [s].[Id0]

SELECT [s].[Id], [s].[Name], [s].[Parent]
FROM [TestEntities] AS [t]
INNER JOIN (
    SELECT [t2].[Id], [t2].[Name], [t2].[Parent], [t0].[Id] AS [Id0]
    FROM [TestEntities] AS [t0]
    CROSS APPLY (
        SELECT [t1].[Id], [t1].[Name], [t1].[Parent]
        FROM [TestEntities] AS [t1]
        WHERE [t1].[Name] IS NOT NULL AND [t0].[Name] IS NOT NULL AND (CHARINDEX([t0].[Name], [t1].[Name]) > 0 OR [t0].[Name] LIKE N'')
    ) AS [t2]
) AS [s] ON [t].[Id] = [s].[Id0]

First, regarding why the outer join is a LEFT JOIN... The fragment inside the outer SelectMany() ends with a DefaultIsEmpty(); this means that for every outer ID, if there's no matching TestEntity, the query should return a null. If the outer query used a non-LEFT INNER JOIN, it would instead filter out any IDs with non-matching TestEntities, which would return wrong result (remember, EF always tries to produce the same results that a regular LINQ-to-Object query would produce).

But more generally, your usage of SelectMany() is slightly odd, and likely inefficient... Taking your original fragmentary query above:

DbSet<MyRecord<long>>().FromSql(select * from unnest({0}), object[] { NpgsqlParameter<MyRecord<long>[]> })
.SelectMany(pk => dataContext.TestItems
    .Where(item => pk.Item == item.ParentId)

What you're looking for here seems to be a simple Contains query:

var ids = new[] { 1, 2, 3 };
_ = context.TestEntities.Where(e => ids.Contains(e.Id))...

This should make the EF provider produce better SQL, using the PG ANY operator, rather than using custom SQL with unnest and SelectMany(), which translates to costly extra joins.

Why is the inner select using cross apply in the first version?
How should we write nested select many, if we need LEFT JOIN for the first level, and LEFT JOIN for the second level?

KvaKoZyaBBrr · 2024-03-28T10:28:11Z

TestEF_selectMany.zip
@roji my bad( I builded wrong repo. Fixed this.
In this examples more true issue.
By actual queries I get

but I need

KvaKoZyaBBrr · 2024-04-08T08:42:06Z

@roji
Can you change to change our linq query to get required sql-query and expected records?

roji · 2024-04-08T08:59:37Z

@KvaKoZyaBBrr I haven't had time to reexamine this. If your repo shows the actual problems this time, I'll try to find some time in the next few weeks and will report back.

KvaKoZyaBBrr · 2024-07-29T14:03:31Z

@roji can I get feedback about this problem or about my PR for current issue please?

roji · 2024-07-29T16:58:21Z

@KvaKoZyaBBrr I'm sorry I haven't been able to look at this - it's a very busy time and it may take a while longer unfortunately. You'll have to be patient a bit more.

dombaigabor · 2024-09-10T05:42:10Z

@KvaKoZyaBBrr I'm sorry I haven't been able to look at this - it's a very busy time and it may take a while longer unfortunately. You'll have to be patient a bit more.

@roji systems like ours, has a lot of data tables on a frontend, powered by SQL views which we would to rewrite into EF Core queries. Now, we have to change the original query syntax if there is a nested join (LEFT then INNER), becasue EF translates it wrong. The problem is the same if there is a LEFT first, than an INNER join.

Please check @KvaKoZyaBBrr's PR if you can.

dombaigabor · 2024-10-07T06:54:31Z

@roji is there any update about this problem? We are waiting for this fix. :(

roji · 2024-10-07T17:19:39Z

I've looked at this again and I'm still not seeing a problem with the SQL that EF is producing - I suspect there are some incorrect expectations here.

@KvaKoZyaBBrr your 2nd code sample is essentially the same as the 1st. I stripped down your query a bit (please do that yourself when submitting code samples, removing anything non-essential):

var selectMany = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => context.TestQueries
                .Where(a => a.Query.Contains(entity.Name))
                .DefaultIfEmpty(),
            (left, right) => new JoinResult<TestEntity, TestQuery>() { Left = left, Right = right }
        ));

As I wrote above, the DefaultIfEmpty() means that a default/empty result needs to be returned for each outer TestEntity; that is why there's a LEFT JOIN and not an INNER JOIN - an INNER JOIN would filter out rows without matches.

I unfortunately can't start explaining how LINQ operators or why EF chooses a specific translation in each specific case. I really, really advise trying to run your LINQ queries in-memory - without EF; look at the results you get, and make sure you understand why those results are returned. If you then see EF returning different results, that would likely be a bug in EF, since EF strives to match the .NET in-memory LINQ evaluation. If you see that happening, please submit a code sample clearly showing the same LINQ query behaving differently - i.e. returning different results - in EF and with in-memory evaluation.

Also, I suggest trying to think about your queries in LINQ, rather than thinking about the SQL you want, and then trying to figure out how to get that exact SQL generated by EF. If you're comfortable with SQL and want to execute a very specific SQL query, there's nothing wrong with just writing that SQL rather than using LINQ. EF really shines when you think in LINQ and let translate to SQL.

I'll go ahead and close this issue for now, so I'm not seeing a problem - yet. If you can submit a LINQ query which behaves differently in EF as it does in-memory (as explained above), I'm happy to reopen and investigate that as a bug.

KvaKoZyaBBrr · 2024-10-07T19:11:11Z

@roji ok let's do it again :)
I have that storage:

DefaultIfEmpty() inside SelectMany() together translate on SQL to LeftJoin. I expect that behavior. And that true.

var firstPart = context.TestEntities
    .SelectMany(x => context.TestQueries
        .Where(a => a.ObjectName.Contains(x.Name))
        .DefaultIfEmpty());

will be translated

SELECT t1."Id", t1."ObjectName"
FROM "TestEntities" AS t
LEFT JOIN LATERAL (
    SELECT t0."Id", t0."ObjectName"
    FROM "TestQueries" AS t0
    WHERE t0."ObjectName" IS NOT NULL AND t."Name" IS NOT NULL AND strpos(t0."ObjectName", t."Name") > 0
) AS t1 ON TRUE

And I get right results. I get some testQueries for any testEntity which contain entityName

Next. I wanna use that case for any child of any testEntity (I will use your version of code)

var selectMany = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => context.TestQueries
                .Where(a => a.ObjectName.Contains(entity.Name))
                .DefaultIfEmpty(),
            (left, right) => new JoinResult<TestEntity, TestQuery>() { Left = left, Right = right }
        ));

Ant this translated to

SELECT s."Id", s."Name", s."Parent", s."Id0", s."ObjectName"
FROM "TestEntities" AS t
LEFT JOIN (
    SELECT t0."Id", t0."Name", t0."Parent", t2."Id" AS "Id0", t2."ObjectName"
    FROM "TestEntities" AS t0
    JOIN LATERAL (
        SELECT t1."Id", t1."ObjectName"
        FROM "TestQueries" AS t1
        WHERE t1."ObjectName" IS NOT NULL AND t0."Name" IS NOT NULL AND strpos(t1."ObjectName", t0."Name") > 0
    ) AS t2 ON TRUE
) AS s ON t."Id" = s."Parent"

And I get

And more funny case if not create testQuery

But it is not what I expect.
I expected
for TestEntity with id=1: 2 childs: id=2 and id=3. For childwith id=2 get 2 query with same name. For child id=3 no query
So for TestEntity with id=1 - 3 rows: 2 filled query and 1 null row (because DefaultIfEmpty is inside inner selectMany!)
for TestEntity woth id=2: 1 child: id=4 witch have 2 query with same name
So for TestEntity with id=2 - 2 row: 2 filled query
for TestEntity with id=3 and id=4 no rows because in that level not contains DefaultIfEmpty
So I expected

because I just create query with selectMany under existed selectMany with defaultIfEmpty.
So how I can get interesting result? I try create sql raw query and I build this (in real I create same query and just formatting to EF style)

So I see what inner leftJoin disapear and transform to innerJoin - that is wrong because inner SelectMany must operate with null. Also I see what outer selectMany which haven`t defaultIfEmpty translated to leftJoin - thats wrong again because outer query does not expect null.
I check EF code and I found problem:
Tranlator visit SelectMany -> deep search DefaultIfEmpty inside arguments -> If DefaultIfEmpty is founded, SelectMany translate to LeftJoin and founded DefaultIfEmpty is disapear.
So I modify EF code in my PR to check DefaultIfEmpty on current level on SelectMany args. I think this is true issue.
Sorry what lot of text, but I work on this problem more 2 weeks and wait answers several months. I really wanna complete this issue :)

attached test project
ClearTest.zip

roji · 2024-10-07T19:13:14Z

@KvaKoZyaBBrr have you tried running your LINQ queries against in-memory collections that contain the same data as in the database, as I suggested above?

roji · 2024-10-07T19:14:36Z

It can really be quite hard to a maintainer like me to understand what exactly you're saying with all of the text and snapshots. It's really easiest if you post a simple console program with a LINQ query that runs once in-memory (no EF at all), and once against a database (with EF). If the results differ between these two executions, then you have a clear bug report with a repro.

I promise that if you post something like this, I'll reopen and we'll investigate.

KvaKoZyaBBrr · 2024-10-07T19:41:50Z

@roji Wow! It`s really very simple to catch problem! Nice!
ClearTest.zip

var firstPart = context.TestEntities
    .SelectMany(x => context.TestQueries
        .Where(a => a.ObjectName.Contains(x.Name))
        .DefaultIfEmpty());
            
var firstPartInMemory = entitiesInMemory
    .SelectMany(x => queriesInMemory
        .Where(a => a.ObjectName!.Contains(x.Name!))
        .DefaultIfEmpty());

thats result are equal!

var selectMany = context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => context.TestQueries
                .Where(a => a.ObjectName.Contains(entity.Name))
                .DefaultIfEmpty(),
            (left, right) => new JoinResult<TestEntity, TestQuery>() { Left = left, Right = right }
        ));

var selectManyInMemory = entitiesInMemory
    .Select(x => x.Id)
    .SelectMany(pk => entitiesInMemory
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => queriesInMemory
                .Where(a => a.ObjectName!.Contains(entity.Name!))
                .DefaultIfEmpty(),
            (left, right) => new JoinResult<TestEntity, TestQuery>() { Left = left, Right = right }
        ));

thats is not equal

vs

KvaKoZyaBBrr · 2024-10-07T19:43:44Z

In-memory result is that I expect on EF

roji · 2024-10-07T20:25:43Z

Confirmed, full minimal cleaned-up repro below. @KvaKoZyaBBrr thanks for doing the work here - in the future a console program like the below is the quickest way of reporting an issue.

var actual = await context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => context.TestQueries
                .Where(a => a.ObjectName.Contains(entity.Name))
                .DefaultIfEmpty(),
            (left, right) => new { Left = left, Right = right }
        ))
    .ToListAsync();

Full minimal repro

await using var context = new BlogContext();
await context.Database.EnsureDeletedAsync();
await context.Database.EnsureCreatedAsync();

List<TestEntity> entitiesInMemory =
[
    new() { Id = 1, Name = "first" },
    new() { Id = 2, Name = "second", Parent = 1 },
    new() { Id = 3, Name = "third", Parent = 1 },
    new() { Id = 4, Name = "fourth", Parent = 2 }
];

context.TestEntities.AddRange(
    new() { Name = "first" },
    new() { Name = "second", Parent = 1 },
    new() { Name = "third", Parent = 1 },
    new() { Name = "fourth", Parent = 2 });

List<TestQuery> queriesInMemory =
[
    new() { Id = 1, ObjectName = "first" },
    new() { Id = 2, ObjectName = "first" },
    new() { Id = 3, ObjectName = "second" },
    new() { Id = 4, ObjectName = "second" },
    new() { Id = 5, ObjectName = "fourth" },
    new() { Id = 6, ObjectName = "fourth" }
];

context.TestQueries.AddRange(
    new() { ObjectName = "first" },
    new() { ObjectName = "first" },
    new() { ObjectName = "second" },
    new() { ObjectName = "second" },
    new() { ObjectName = "fourth" },
    new() { ObjectName = "fourth" });

await context.SaveChangesAsync();

var actual = await context.TestEntities
    .Select(x => x.Id)
    .SelectMany(pk => context.TestEntities
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => context.TestQueries
                .Where(a => a.ObjectName.Contains(entity.Name))
                .DefaultIfEmpty(),
            (left, right) => new { Left = left, Right = right }
        ))
    .ToListAsync();

var expected = entitiesInMemory
    .Select(x => x.Id)
    .SelectMany(pk => entitiesInMemory
        .Where(entity => pk == entity.Parent)
        .SelectMany(
            entity => queriesInMemory
                .Where(a => a.ObjectName!.Contains(entity.Name!))
                .DefaultIfEmpty(),
            (left, right) => new { Left = left, Right = right }
        ))
    .ToList();

Console.WriteLine($"Expected: {expected.Count}, actual: {actual.Count}");

public class BlogContext : DbContext
{
    public DbSet<TestEntity> TestEntities { get; set; }
    public DbSet<TestQuery> TestQueries { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer("Server=localhost;Database=test;User=SA;Password=Abcd5678;Connect Timeout=60;ConnectRetryCount=0;Encrypt=false")
            .LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging();
}

public class TestEntity
{
    public int? Id { get; set; }
    public string? Name { get; set; }
    public int? Parent { get; set; }
}

public class TestQuery
{
    public int? Id { get; set; }
    public string? ObjectName { get; set; }
}

@maumar does this ring any bells? Interested in looking into it?

KvaKoZyaBBrr · 2024-10-07T20:31:03Z

@roji I tryed resolve this in PR
#34110
Is this may be usefull?

roji · 2024-10-07T20:33:28Z

Thanks again @KvaKoZyaBBrr, we'll take a look! The PR definitely looks interesting.

KvaKoZyaBBrr added the customer-reported label Mar 18, 2024

AndriySvyryd added the area-query label Mar 20, 2024

AndriySvyryd assigned roji Mar 20, 2024

KvaKoZyaBBrr linked a pull request Jun 28, 2024 that will close this issue

Improved nested SelectMany with DefaultIfEmpty #34110

Open

6 tasks

roji closed this as not planned Won't fix, can't repro, duplicate, stale Oct 7, 2024

roji added the closed-no-further-action The issue is closed and no further action is planned. label Oct 7, 2024

roji reopened this Oct 7, 2024

roji added type-bug and removed closed-no-further-action The issue is closed and no further action is planned. labels Oct 7, 2024

maumar self-assigned this Oct 8, 2024

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Why DefaultIfEmpty inside double SelectMany is wrong working? #33343

Why DefaultIfEmpty inside double SelectMany is wrong working? #33343

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Why DefaultIfEmpty inside double SelectMany is wrong working? #33343

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