Generate table joins instead of subquery joins

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
38/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp
Domain
database

Research direction

The issue names no files or tests, but provides two SQL shapes for queries using includes. Start by locating EF Core's includes query-generation path, then measure execution performance for both forms; done means a documented decision and, if adopted, generated SQL matching the simpler join structure.

Written by the indexing model from the issue text.

Description

area-perf area-query

For queries with includes, we currently generate joins with a subquery:

SELECT [b].[Id], [b].[Name], [b].[UserId], [t].[Id], [t].[BlogId], [t].[Description], [t].[UserId], [t].[Id0], [t].[Created], [t].[Hash], [t].[IsDeleted], [t].[Modified], [t].[PostId]
FROM [Blog] AS [b]
LEFT JOIN (
    SELECT [p].[Id], [p].[BlogId], [p].[Description], [p].[UserId], [p0].[Id] AS [Id0], [p0].[Created], [p0].[Hash], [p0].[IsDeleted], [p0].[Modified], [p0].[PostId]
    FROM [Post] AS [p]
    LEFT JOIN [PostInstance] AS [p0] ON [p].[Id] = [p0].[PostId]
) AS [t] ON [b].[Id] = [t].[BlogId]
ORDER BY [b].[Id], [t].[Id], [t].[Id0]

We could simplify this to:

SELECT [b].[Id], [b].[Name], [b].[UserId], [t].[Id], [t].[BlogId], [t].[Description], [t].[UserId], [t].[Id0], [t].[Created], [t].[Hash], [t].[IsDeleted], [t].[Modified], [t].[PostId]
FROM [Blog] AS [b]
LEFT JOIN [Post] AS [p] ON [b].[Id] = [p].[BlogId]
LEFT JOIN [PostInstance] AS [p0] ON [p].[Id] = [p0].[PostId]
ORDER BY [b].[Id], [p].[Id], [t].[Id0]

We should measure the execution perf difference between the above two. Even if there is no (significant) difference, we could still decide to do this for SQL simplicity.

Originally raised in https://github.com/aspnet/EntityFrameworkCore/issues/17455.

Dominant language
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