[xperiandri]

Draft, do not merge. SQLProvider 1.5.27 finds a left join by the DefaultIfEmpty inside the GroupJoin result, which is what F# emits today. With this PR every leftOuterJoin there silently becomes an INNER JOIN (16 queries and 8 of its own tests), and EF Core stops translating a group that is both iterated and read again. Kept for discussion of a design that works for both, for #6552.

Stacked on #20902

Two leftOuterJoins in a row now become two SQL LEFT JOINs: the group reaches the rest of the query as GroupJoin produced it, and DefaultIfEmpty is applied in the SelectMany that reads it, the shape C# produces and EF Core and linq2db recognize as a left join.

Details

• leftOuterJoin used to apply DefaultIfEmpty inside the GroupJoin result, so the join's element carried an already-defaulted group. With a second leftOuterJoin after it EF Core needed a lateral join (APPLY), which SQLite lacks, and linq2db failed.
• The elements are the same, but the group is no longer one object per joined row: each read makes its own DefaultIfEmpty sequence. A query that compares the group itself by reference over LINQ to Objects, such as select g; distinct or groupBy g, sees the difference.

🤖 Generated with Claude Code

[github-actions]

✅ Release notes checked

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/FSharp.Core`	docs/release-notes/.FSharp.Core/11.0.200.md

[xperiandri]

Covered cases

EF Core SQLite translates 102 of 139 scenarios, 100 on #20902. Now translated: two left joins spelled leftOuterJoin, and a join after a left join.

linq2db translates two leftOuterJoins too; it fails on the second today. EF Core for SQL Server and PostgreSQL produces two LEFT JOINs where it used a lateral join.

Why this is a draft

• SQLProvider 1.5.27 on SQLite: every leftOuterJoin becomes an INNER JOIN without an error, so rows with no match go missing (16 queries and 8 of its own tests). It sets isLeftOuter from a DefaultIfEmpty inside the GroupJoin result selector, and treats a DefaultIfEmpty in the SelectMany collection selector as a no-op, which is the C# shape this PR produces.
• EF Core: leftOuterJoin o in db.Orders on (…) into orders; for o in orders do select (c.Name, orders.Count()), a group that is both iterated and read again, translates on main and no longer does.