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Query: Add support for GroupBy patterns beyond aggregate
- Allow expanding navigations after GroupBy operator applied before reducing it to non-grouping Fixes#22609
- Translate FirstOrDefault over grouping element Fixes#12088
- Add ability to select N element over grouping element Fixes#13805
Overall approach:
A grouping element (the range variable you get after applying GroupBy operator) is of type `IGrouping<TKey, TElement>` which implements `IEnumerable<TElement>`. Hence we treat this enumerable as if it is queryable during nav expansion phase. During translation phase we inject ShapedQueryExpression in place of the grouping element which is being enumerated. What this allows us is to expand navigation just like any other query root and translate a subquery similar to other subqueries to facilitate reusing same code for the tasks.
During translation phase in relational layer, since aggregate operation can be lifted into projection for SelectExpression containing SQL GROUP BY. This code path works in 2 ways, when translating we try to combine predicate/distinct into the aggregate operation (so in future when we support custom aggregate operators, we don't have to understand the shape of it to modify it later. When adding this scalar subquery to SelectExpression, we try to pattern match it to see if we can lift it. Further during lifting, we also lift any additional joins in the subquery (which implies there were some joins expanded on grouping element before aggregate) including the navigation expanded from owned navigations. A pending TODO is to de-dupe navigation expanded. It is not straight forward since aliases of table would have changed when previous was lifted.
Given every enumerable grouping element act as query root, every time we replace it inside a lambda expression, we need to create a copy of the root. Navigation expansion and individual queryableMethodTranslatingEV does this. So each root act and translate independently from each other.
Bug fixes:
- Fix a bug in identifying single result in InMemory to convert it to enumerable
- Null out _groupingParameter in InMemoryQueryExpression once the projection to reduce it has been applied
- Throw better error message when translating Min/Max over an entity type for InMemory
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