Re: A real world example


"erk" <eric.kaun@xxxxxxxxx> wrote in message
news:1155908080.655605.22860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Brian Selzer wrote:
"erk" <eric.kaun@xxxxxxxxx> wrote in message
news:1155827594.752249.65890@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Brian Selzer wrote:
[snip]
If a constraint is defined in terms of successive states of a database,
then
facts cannot be thought of just in terms of instances of predicates.

Yes, that's all facts are, though constraints mean that your facts are
consistent with one another, based on how you've defined the predicates
of relevance to you (i.e. inter-relation constraints reflect
requirements as well as "reality"). I'm speaking out of some degree of
ignorance; no database I've ever worked with has had state-transition
constraints like this. The only constraints have been what constitutes
a valid relation (and database) value.

Any such instance has identity only within a single relation value.
Since the
value of a candidate key determines the values of all other attributes,
it
can be used to identify a single tuple within a single relation value;
therefore, it can be used in other relation values as a substitute for
enumerating all of the attribute values of the referenced tuple in every
referencing tuple within the same database state--but only within the
same
database state.

I don't think that's a valid interpretation. It's not a substitute for
referencing all of the attribute values, nor, given only the key value,
can I determine the values of those attributes. The key has a specific
meaning in all the predicates in which it's referenced, as distinct
from the other attributes - it's not a surrogate for them. Relations,
even those with foreign key constraints, are standalone facts. Joins
and constraints reflect references to the same types.

I think this difference is important, and am not just spitting hairs
for the sake of it.

Extending the scope of a candidate key's ability to
identify instances of predicates from a single database state to
successive
database states would require that those instances be identical, not just
the candidate key values.

So this ability to uniquely identify a fact across successive database
states is important purely for state-transition constraints? Is there
any other use for them?

If the values determined by the candidate key
value in the proposed state were different than those in the current
state,
then the instances would not have the same properties, and therefore,
would
not be the same.

Sameness is irrelevant; a fact is a fact, and if database value N+1
differs from database value N, it's because facts have changed, and we
need the database to reflect reality more accurately.

Instances are values; values do not change. Therefore,
relaxing this restriction so that a candidate key value can identify
instances in successive database states that are not necessarily
identical,
but have identical candidate key values can only be possible if the
instances of a predicate represent things in the universe of discourse
that
can have their appearance altered without altering their identity, and
what
is identified by a candidate key value is not just a tuple, but something
in
the universe.

It's a fact, not a thing. I have to admit that I'm still quite leery of
the notion that facts have "identity." I have no current
counterproposal; I'm just trying to understand why this is necessary. A
fact is a statement about things. One attribute of such a statement
might be that a real-world thing has a real-world unique identifying
value, and I understand the need to sometimes introduce surrogates. But
given that a separate "identity attribute" for a fact can have no
possible correlation with anything in the real world, including natural
candidate keys, I smell potential problems.

Maybe, but from a functional standpoint, that operator is just a
function (e.g. "subtract $500 from X), in which the balance is a
free
variable. Only in an imperative world does that involve "knowing"
(referencing) the "previous" balance. Function application means
there's no "query" of the value prior to the update.

Not necessarily. For example, consider a sales order that can have
several
states, proposed, open, firm, shipped, received, closed, cancelled.
Assume
that the order stated is the normal set of state changes for the
order.
Now
consider that an order that cannot become proposed once it is firm, it
cannot become received unless it has been shipped. It cannot become
closed
unless it has been received. Unless you define special operators to
deal
with the states, you need to know what the old value was in order to
maintain the consistency of the database throughout the update.

Maybe the discrepancy hinges on the phrase "you need to know." I'd
argue that no query is needed, merely constraints.

What type of constraints? I don't understand how you could define a
constraint. Could you please show me?

Sorry, my comment didn't directly address the issue, so I'll rephrase:
given that an application can produce several different state
transitions in a row, I'm not sure what value these state-transition
constraints would have. For example, an application can issue two
updates in a row: set order status to open, then immediately set to
firm. Such state transition constraints seem to have no meaning at all
if they don't reference other relations (e.g. you can't set the status
to received if there's no corresponding fact regarding the receipt
date/time), aren't they just "static" relation/database constraints,
which can be enforced for every update?

I have to give this much more thought, but is this a case where
syntactic sugar for state-transition constraints could simply
transparently introduce "surrogate relations" to implement the state
transition constraints as "static" database constraints?

More
importantly, the bank must be able to identify the account that is
about
to
change, and that identity must remain constant in both the
preceding
and
succeeding database instances.

Why? As long as it can be identified via some query, what difference
does it make? For example, if I make a database schema change and
introduce a new key, with appropriate view changes to support old
application code, is there some logical distinction? If the external
queries all still produce the same results, excepting the specific
values being updated, what does "identity" have to do with it?

Because changes are set-based, and if the identity of the account can
change, then it's possible to update the wrong row, or to allow a
charge
to
clear that shouldn't be allowed.

There is no "wrong row," only a set of propositions. The same
possibility for human error would seem to be present in any update:
that you might issue an update without knowing about a change made
between the time you last loaded the page, and the time you pressed
Save, and therefore could violate a constraint which you wouldn't
violate if only the database were in the state you think it is (based
on what's on the screen). This issue seems to be a particular variant.

There is no "thing." These are propositions, or assertions if you
like,
nothing more. The only meaning is in the correlation of queries to
external phenonema of interest.

What are the propositions or assertions about? If they're about
values
then
they're just hot air. A database contains knowledge. Knowledge about
what?
Scalar values? I don't think so.

They're about what is in our heads - the application (business) domain.
The database doesn't care about that; it's in crafting predicates and
constraints that we tell the database as much as it needs to (or can)
"know."

The relational model doesn't have a correct theoretical mechanism to
correlate tuples during updates. The scope of a key value's ability
to
identify a tuple is a single relation value from a single database
instance.
I think that the model is incomplete without such a mechanism, because
there
are some constraints that cannot be enforced, and certain update
anomalies
can occur, as I've provided examples of in other posts.

Since we're not talking about a machine that "really knows" the real
world, I don't understand what sort of mechanism you have in mind -
what is an example of a "correct theoretical mechanism"? The relational
model already allows surrogate keys.

But it does not require them. Nor does it define mutability constraints
in
conjunction with entity integrity. Nor does it define a tuple-level
assignment operator.

Aren't tuple-level assignment operators unnecessary if you have the
surrogate keys you seek?


The problem is that none of them are defined in the model. If the model
even stated "there must exist some means to correlate tuples during an
update," then every implementation of the model must provide that
capability. There may be other ways to correlate tuples that I haven't
enumerated or even conceived. Whether it's object identifiers, tuple
identifiers, mutability constraints in the context of entity integrity,
tuple-level assignment, redefining the assignment operator to be three
separate operators that accept ordered sets instead of sets, or something
else, is less important than recognizing that the need exists. The
discussion about which is the best solution is secondary.

In any event, I'm not sure that mandatory surrogate keys solve more
problems than they create. The ability to change the non-surrogate keys
arbitrarily seems to indicate that The anything, given that they allow
arbitrary combinations of the various keys. In other words, by
definition the surrogate key is unrelated to anything else, and as such
it seems the keys can be shuffled at will. In the case of a relation
where there is only a single surrogate key (e.g. the tuples represent
facts that are nearly indistinguishable, like events in a trace), it
doesn't matter.

I think that the definition of the model should be
strong enough so that I can't break it.

I think you really expect way too much from models, and I'm not sure
this is a fracture any worse than the cure would produce. I think these
are fundamental identity problems, not just ones particular to the
relational model.


I don't think that expecting constraints to be enforcable is expecting too
much.

At the risk of being accused of waving my hands, I'll quote from one of
Bill Kent's papers, The Unsolvable Identity Problem
(http://www.idealliance.org/papers/extreme/proceedings/html/2003/Kent01/EML2003Kent01.html):

"Why is the identity problem unsolvable? To begin with, as just shown,
we don't agree on what the identity problem is."

"A general unified theory of identity is elusive. It probably doesn't
exist. The main reasons:
* The problem is not well defined.
* There are theoretical and practical limitations to what can be
achieved.
* There are too many semantic issues.
* There are too many domains. We can't achieve a consistent
solution across all of them.

But this quest for the Holy Grail is educational."

"So what do we do? Cope, as we always do. If there is no ideal
solution, we develop solutions that are good enough. The trouble is
that what's good enough for you today isn't good enough for me
tomorrow. We are forever doomed to compromise, extend, patch and rework
to make our good enough solutions a little better. We'll never get it
right. That's life.

Human beings manage to cope somehow with imperfect identification
schemes. Our computer systems might do no better than that."

- erk



.

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