Re: A pk is *both* a physical and a logical object.
Date: Fri, 13 Jul 2007 17:37:42 GMT
"paul c" <toledobythesea_at_oohay.ac> wrote in message
> David Cressey wrote:
> > Specifically, does Tutorial D have the concept of "primary key" in it?
> > so, how does it relate to relational theory? If not, how does it
> > the same problems that lead SQL practitioners to lean on the concept of
> > primary key?
> Here's a partial quote from TTM, 2nd edition, RM prescription 15 (sorry
> in advance for any typing errors of mine):
> "Note. Historically, it has been usual to insist that, at least in the
> case of real relvars, there be a distingished candidate key called the
> primary (italics) key. While this discipline might be useful in
> practice, we do not insist on it (and Tutorial D as defined in Chapter 5
> does not support it), because we regard the idea of making one candidate
> key somehow "more equal thant the others" as a psychological issue
> merely . Of course, we do not prohibit it either."
Thanks. I think there is a better word out there than "psychological", but I can't settle in one.
Your quote confirms my percetion of the relationship between primary keys and the relational model of data.
> ( refers to a CJ Date article - "The Primacy of Primary Keys: An
> Investigation.) TTM also repeats the usual properties, uniqueness and
> irreducibility and since the above is a TTM "prescription", I presume it
> goes beyond Tutorial D, whereas the A-algebra has no reference to keys
> of any kind.
And, I am told, the SQL standard does not mention indexes of any kind.
> Codd's 1970 paper is full of the term, but I find it's often echoing the
> IMS term of the same name, IMS being a hierarchical dbms that I was told
> pretty much everybody at IBM labs was paid to know and study. Here's
> some of what Codd had to say:
Thanks again. It's important to note, when looking over the 1970 paper, that Codd was explaining the unfamiliar by connecting back to the familiar, a very common approach to explaing new things.
> "1.2.2. Indexing Dependence. In the context of formatted
> data, an index is usually thought of as a purely
> performance-oriented component of the data representation.
> It tends to improve response to queries and updates
> and, at the same time, slow down response to insertions
> and deletions. From an informational standpoint, an index
> is a redundant component of the data representation. If a
> system uses indices at all and if it is to perform well in an
> environment with changing patterns of activity on the data
> bank, an ability to create and destroy indices from time to
> time will probably be necessary. The question then arises:
> Can application programs and terminal activities remain
> invariant as indices come and go?"
> (This point about a dependence that he wished to eliminate followed one
> about Ordering Dependence.)
> I remember IMS had another "key" called a "secondary key". Certainly
> years ago I know most programmers thought of it as a very clumsy index,
> usually added when an unforeseen query, what I think Codd called a
> symmetrical one, reared its ugly face. This all leads me to think that
> the "primary" adjective first appeared in either IBM's or some other
> vendor's hierarchical dbms'es. Whereas the term "key" was well
> entrenched from the late 1950's, when every programmer had to know
> peripheral hardware and be able to write programs that are today called
> device drivers. Lynn W who visits here sometimes might clarify my
> memory but as I recall, some tape drives had physical count blocks and
> when disks such as the Ramac came along, "key" physical blocks were
> added and some of the "ISAM" support was built into the hardware
> channels and devices, for example, cylinder searches. Whatever, it's
> for sure that programmers and everybody around those machines treated an
> index as synonomous with a key although the reverse wasn't always the
> case because some keys were direct pointers and some were hashes. I'm
> sure Honeywell, Burroughs, Sperry etc., had similar gizmos. (I think it
> was around this time that DEC and Wang were about to put the "BUNCH" of
> IBM competitors out of business, don't know much about their early
> machine peripherals.)
Just a historical side note: BUNCH was an acronym (or perhaps merely a hackronym.) It stood for Burroughs, Univac, National Cash Register, Control Data, and Honeywell. None of them were put out of business by DEC or Wang, as I recall. Rather, they were insufficiently differentiated from IBM to withstand the market pressure that IBM placed on them via their customers. I can't speak for Wang, but I would say DEC survived for as long as it did by appealing to subset of customers that had always been difficult for IBM to serve.
I remember in the 70s and 80s a large number of sites that has both IBM gear and DEC gear in their data center.
> Just my worm's-eye view. That's enough memory lane for me for today.
\ Received on Fri Jul 13 2007 - 19:37:42 CEST