TOP(1) BY GROUP of very huge (100,000,000+) table
If you have a table of SourceID values, and an index on your main table on (SourceID, End) include (othercolumns), just use OUTER APPLY.

SELECT d.*
FROM dbo.Sources s
OUTER APPLY (SELECT TOP (1) *
    FROM data.OperationData d
    WHERE d.SourceID = s.SourceID
    ORDER BY d.[End] DESC) d;


If you know you’re only after your newest partition, you could include a filter on End, like AND d.[End] &gt; DATEADD(day, -1, GETDATE())

Edit: Because your clustered index is on SourceID, Source, End), put Source into your Sources table too and join on that as well. Then you don’t need the new index.

SELECT d.*
FROM dbo.Sources s -- Small table
OUTER APPLY (SELECT TOP (1) *
    FROM data.OperationData d -- Big table quick seeks
    WHERE d.SourceID = s.SourceID
    AND d.Source = s.Source
    AND d.[End] &gt; DATEADD(day, -1, GETDATE()) -- If you’re partitioning on [End], do this for partition elimination
    ORDER BY d.[End] DESC) d;