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Size is in play, yes, but the 8 extra bytes per row is likely negligible compared to the row size. Is there a case where the dise matters? Sure. But you can't discuss the space cost for 10 billion rows without comparing to the space cost of 10 billion rows.

SQL server let's you cluster by any index, do if your child record table will benefit by clustering by ParentGuid then go for it.




MySQL stores a copy of the PK in every secondary index, so it can start adding up quite a bit. I agree that the overall size of 10 billion rows would dwarf that, but since you're presumably doing some decent indexing on a table of that size, index size matters more IMO.

For any RDBMS (I assume... I don't know a lot about SQL Server or Oracle), the binpacking for pages also impacts query speed for queries where there are many results.




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