Query: collection join scenario where inner collection is a group by correlated with outer should be detected and blocked

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp
Domain
database

Research direction

Start with the collection-join processing described in the issue and reproduce the provided LINQ example. Check that a correlated inner GroupBy is detected and blocked without producing invalid SQL, while an uncorrelated GroupBy subquery continues to work.

Written by the indexing model from the issue text.

Description

area-groupby area-query

Example:

 ctx.Customers.Where(c => c.CustomerID.StartsWith("A"))
    .Select(c => c.Orders.GroupBy(o => o.OrderID).Select(g => g.Key).ToArray());

when processing collection join we try to add correlation predicate columns to the inner projection, however that produces incorrect sql - group by query can only project grouping keys (and aggregates).
Outer key can't be part of the grouping either (Each GROUP BY expression must contain at least one column that is not an outer reference.)

Uncorrelated group by subqueries should still work.

Dominant language
C#
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