Reuse a single-result subquery across its projected members#38502
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…otnet#7776) A projection that reads two or more members of one correlated First/Single/ Last/ElementAt subquery now lifts to a single SelectMany during navigation expansion, so the members come from a single join instead of a duplicated subquery per member.
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Pull request overview
This PR introduces a targeted navigation-expansion rewrite to avoid duplicating correlated single-result subqueries when multiple members are projected from the same subquery (e.g. c.City, c.Country, c.ContactName), by lifting the subquery into a single join/SelectMany shape so all projected members come from the same joined row.
Changes:
- Add
LiftSingleResultSubqueriesto rewrite repeated member-access projections over the same correlated single-result subquery into a singleSelectMany. - Invoke the lifting pass after
ProcessSelectinNavigationExpandingExpressionVisitorforQueryable.Select. - Add a cross-provider spec test plus SQL Server SQL baseline coverage; Cosmos asserts translation failure.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.
Show a summary per file
| File | Description |
|---|---|
| src/EFCore/Query/Internal/NavigationExpandingExpressionVisitor.SingleResultSubqueryLifting.cs | New lifting pass to detect repeated member accesses over correlated single-result subqueries and rewrite to a single join via SelectMany. |
| src/EFCore/Query/Internal/NavigationExpandingExpressionVisitor.cs | Hooks the lifting pass into the Queryable.Select handling path. |
| test/EFCore.Specification.Tests/Query/NorthwindSelectQueryTestBase.cs | Adds the new scenario as a provider-agnostic spec test across single-result operators. |
| test/EFCore.SqlServer.FunctionalTests/Query/NorthwindSelectQuerySqlServerTest.cs | Adds SQL Server baseline assertions verifying the lifted single-join translation shape. |
| test/EFCore.Cosmos.FunctionalTests/Query/NorthwindSelectQueryCosmosTest.cs | Marks the new test as translation-failing for Cosmos. |
AndriySvyryd
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Jun 30, 2026
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I've put together a small interim fix for #7776 and wanted to share it for your consideration. I'd completely understand if you'd rather wait for the pending-selector rework in #20291 (which you've noted #7776 is probably a duplicate of). But since that's a larger effort, a self-contained stopgap might be useful in the meantime.
I've kept it intentionally scoped to a single ~100-line file, plus a one-line call in
NavigationExpandingExpressionVisitor. Easy to replace in the future when the root fix lands.It fires only when a projection reads two or more members of one correlated single-result subquery, and rewrites that into a
SelectManyso the members come from one join. Everything else is left untouched. It doesn't address the general reused-subquery case (#20291 would), only this shape.Tested across the single-result operators (
First/Single/Last/ElementAtand theirOrDefault/predicate forms) on SQL Server, SQLite and the in-memory provider; the full SQL Server suite passes with no baseline changes.