Relational: Translate SelectMany

[smitpatel]

SelectMany in linq look something like following (second clause is considered the collection selector)

from c in cs
from o in c.Orders
select....

Following are the translations of SelectMany

Unrelated collection selector

from c in cs
from o in os

Generates CROSS JOIN

Correlated collection selector with correlation being a predicate

from c in cs
from o in os.Where(o => o.CustomerID == c.CustomerID)

Such predicate can be lifted and used in generating a join. So this query generates JOIN.
If collection selector ends with DefaultIfEmpty then it is LEFT JOIN.

Correlated collection selector with correlation not a predicate

from c in cs
from o in os.Select(o => c.City)

Since we cannot generate a join predicate here, this translates to CROSS APPLY.
If collection selector ends with DefaultIfEmpty then it is OUTER APPLY.

• Add support for Cross Apply
• Add Support for Outer Apply
• Convert Cross Apply to Inner Join when possible
• Convert Outer Apply to Left Join when possible
• Add translation for DefaultIfEmpty
• Add translation for both overloads of SelectMany
• Handle DefaultIfEmpty & SelectMany without collectionSelector in Navigation Expansion
• Ensure columns are in projection when generating join predicate from a correlated subquery

Currently there are no tests for Cross/Outer Apply.
Our earlier Cross Apply got converted to join. N+1 evaluation tests are disabled right now, which would generate Cross Apply.
We never supported Outer Apply in past.

Resolves #15711
Resolves #12567
Resolves #12572
Resolves #12872
Resolves #16330
Resolves #15081
Resolves #16989

Re-enable tests for #12449

[ajcvickers]

@maumar Please make it a priority to review this when you get in. (You were nominated in triage! 🚎)

[smitpatel]

Renamed CrossApplyExpression -> InnerJoinLateralExpression & OuterApplyExpression -> LeftJoinLateralExpression to match with formal SQL terminology. Also updated relational implementation and moved SqlServer specific sql syntax in provider code.