The theory, indeed, does not say anything about buffer pools, but by decoupling logic
from implementation we leave the implementor (DBMS) to do as it feels fit to do.
As DBMS technology advances, we get faster systems without having to change our
programs.
I think you've identified why relational systems have been the overwhelming winner in the business environment. They allow vendors to provide an optimized but fairly general solution to the interesting problem of efficiently accessing and storing data on rotating magnetic storage, while at the same time presenting a programming model that's at just the right level for the business applications programmer.
Relational theory or no, linked tables are typically conceptualized as a slight formalization of the spreadsheet, or (in earlier times) stacks of punched cards. As business computers evolved from more specific machines that could perform some relational operations on punched cards (sort, select, etc.), I think it's still sort of stuck in the collective unconscious of business to want to model their data this way.
I think relational theory is useful primarily to database implementers, students, and those few application developers who are after a deeply theoretical understanding of their tools. They're probably the ones reading this list.
I suppose MV and other non-SQL data stores have their place in a certain niches (embedded systems, etc.), but the business world has already voted with it's feet.
- Marsh
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