[release/11.0] Restore LEFT JOIN translation for correlated grouped DefaultIfEmpty

[Copilot]

Fixes #39220

Description
Since 10.0-rc.2, correlated SelectMany over a grouped subquery followed by DefaultIfEmpty() can produce OUTER APPLY instead of LEFT JOIN even when the key nullability doesn't match.

Customer impact
Affected queries are translated to OUTER APPLY, which causes providers without APPLY support (including SQLite and MariaDB) to throw instead of running the query. An explicit join … into … from … DefaultIfEmpty() is a workaround, but it's non-obvious and rewriting a complex query can require significant work.

Example of an affected query:

var grouped = from t in db.Transactions
              group t.Amount by t.UserId into g
              select new { g.Key, Amount = g.Sum() };

var query = from u in db.Users
            from t in grouped.Where(t => t.Key == u.Id).DefaultIfEmpty()
            select new { u.Id, Amount = (int?)t.Amount ?? 0 };

How found
Customer-reported on 10.0.12.

Regression
Yes, from 9.0.x.

Testing
Tests added

Risk
Low. The change is a narrow condition that reverts to previous behavior. Quirk mode added.

[Copilot]

🟡 Changes recommended

The servicing PR description omits the required customer impact, regression, testing, and risk sections.

0 open findings

What changed in this PR

Restores LEFT JOIN translation for correlated grouped or distinct DefaultIfEmpty queries while retaining a servicing opt-out.

Changes:

• Accepts nullable projected join keys.
• Adds shared behavioral tests and provider SQL baselines.
• Documents the query-pipeline nullability behavior.

File	Description
SelectExpression.cs	Updates join-key matching and adds the compatibility switch.
NorthwindGroupByQueryRelationalTestBase.cs	Adds shared GroupBy and Distinct tests.
NorthwindGroupByQuerySqlServerTest.cs	Verifies SQL Server LEFT JOIN SQL.
NorthwindGroupByQuerySqliteTest.cs	Verifies SQLite LEFT JOIN SQL.
query-pipeline/​SKILL.md	Documents nullable column identity handling.

🧠 Review effort: Balanced

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[artl93]

Customer reported regression. Approved.

[Copilot]

🟢 Approval recommended

The narrowly scoped fix is covered by result assertions and provider-specific SQL baselines.

0 open findings

🧠 Review effort: Balanced

[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage

The overall line coverage in commit dce40ae in the copilot/fix-regressi... branch is 67%. The line coverage in commit 2c982ae in the release/11.0 branch is 76%.

Show a line coverage summary of the most impacted files.

File	release/11.0 2c982ae	copilot/fix-regressi... dce40ae	+/-
/Users/runner/w...nal/Snapshot.cs	0%	60%	+60%
D:\a\efcore\efc...nal\Snapshot.cs	0%	64%	+64%
/home/runner/wo...nal/Snapshot.cs	0%	66%	+66%
/home/runner/wo...yTypeBuilder.cs	0%	88%	+88%
D:\a\efcore\efc...yTypeBuilder.cs	0%	88%	+88%
/Users/runner/w...ngs.Designer.cs	0%	92%	+92%
/home/runner/wo...ctExpression.cs	0%	93%	+93%
D:\a\efcore\efc...ctExpression.cs	0%	93%	+93%
D:\a\efcore\efc...ngs.Designer.cs	0%	94%	+94%
/home/runner/wo...ngs.Designer.cs	0%	95%	+95%