[seragAhmed]

Fixes #39628

The lift of GroupBy aggregates over reference navigations (#38668) is all or nothing: GroupingAggregateScanner only recognizes aggregates applied directly to the grouping, and any other use of the grouping makes TryLiftAggregatesOverNavigations give up for the whole projection. So one g.Select(x => x.Value).Distinct().Count() next to g.Max(x => x.Author.Rating) turned every navigation aggregate back into a correlated subquery.

The scanner now also accepts a short operator chain before the aggregate: Where over the element, at most one Select, and Distinct after the Select. The lambdas over the element are expanded on the parent like aggregate selectors, and the chain is rebuilt over the lifted grouping. After a Select, only aggregates without a selector or predicate are accepted (including the non-generic numeric overloads such as Sum(IEnumerable<int?>)), since a lambda over the projected values would not have its navigations expanded. Other uses of the grouping keep the existing translation.

ctx.Orders.GroupBy(o => o.EmployeeID).Select(g => new
{
    g.Key,
    Region = g.Max(o => o.Customer!.Region),
    Customers = g.Select(o => o.CustomerID).Distinct().Count()
});

-- before: MAX through a correlated subquery; after:
SELECT [o].[EmployeeID] AS [Key], MAX([c].[Region]) AS [Region], COUNT(DISTINCT ([o].[CustomerID])) AS [Customers]
FROM [Orders] AS [o]
LEFT JOIN [Customers] AS [c] ON [o].[CustomerID] = [c].[CustomerID]
GROUP BY [o].[EmployeeID]

Two cases are deliberately left alone:

• Distinct over a navigation is not lifted. The correlated translation counts the null of a missing principal as a value (COUNT(*) over SELECT DISTINCT), while the lifted one would use COUNT(DISTINCT ...) and skip it. Group_by_multiple_aggregate_joining_different_tables keeps its baseline.
• For a principal relaxed to an outer join because of its query filter (GroupBy aggregate over a required navigation with a query filter drops entire groups (Regression in 11.0) #38965), an aggregate with a chain is always guarded, since Where, Select and Distinct can observe the null-extended row.

Parameter_collection_Contains_with_projection_and_ordering uses g.Select(e => e.Order.OrderDate).Max(), which is now lifted too; its SQL Server baseline is updated (one INNER JOIN instead of two correlated subqueries).

Tests:

• NorthwindGroupByQueryTestBase: distinct count, select-then-aggregate and where-then-count next to navigation aggregates (lifted), and a predicate after Select (still correlated), with SQL Server baselines.
• AdHocQueryFiltersQueryRelationalTestBase: the three chain kinds over a required navigation to a filtered principal, with SQL Server and SQLite baselines.

Locally, the SQL Server and SQLite Microsoft.EntityFrameworkCore.Query* functional tests, the InMemory GroupBy tests and EFCore.Tests pass.

🤖 Generated with Claude Code

[seragAhmed]

@dotnet-policy-service agree

[seragAhmed]

@dotnet-policy-service agree