To me, drop-in replacement means the interface is the same, not implementation. You can have list operations on top of array, single or double linked lists etc.

To me, drop-in replacement means the interface is the same, not implementation. You can have list operations on top of array, single or double linked lists etc., and you'll get different Big O numbers for different operations, different memory access patterns - but will still have the same append() and length() functions in the API. Drop-in doesn't mean "equivalent in all measurable aspects". I spend substantial time navigating and compensating for differences in DB interfaces - not characteristics of internal solutions. Standards aren't about that - they are, as I understand, to solve the problem which I have, because standards aren't followed. I want to be able to replace databases not because they are the same - what would be the purpose then - but because having the same interface (i.e., the ability to just replace the DBMS) they could provide me a choice of characteristics.