header part of the value?
Date: Sun, 24 Feb 2008 10:48:51 -0800 (PST)
Occasionally the question has come up as to whether a relation value is the body, or the body+the header. In the past I've sided with the just-the-body approach, but today I decided that I don't think that anymore.
Consider the algorithm to perform a natural join on two relation values. Just values: not tables in a database with a known schema or whatever. Just two plain relation values. The natural join specification *requires* the header; it is defined (in part) in terms of the header. So the header must be part of the value.
Of course, that then leads me to think of a relation value as a <header, body> tuple. (Let us omit column type information for the present discussion.) Then one imagines an updatable relation variable in a database as holding a value of this tuple type. BUT then we notice that we have this restriction that the header must not be updated. Why is that?
Certainly in practice this is the sort of thing that would be almost universally a good idea. But what theoretical basis does it have? I can think of none. So I propose, for your amusement, the mental model that a relation variable is merely a simple binding from a name to a <header, body> relation value, period, full stop. Also, *customarily* the variable has the update constraint that old.header = new.header.
Marshall Received on Sun Feb 24 2008 - 19:48:51 CET