Subject: Proposed Glossary Entries (History and related terms)

From: (Fabio Grandi) <[email-masked]>
Date: Fri, 7 May 93 15:18:17 +0100

We tried to complete our previous entry proposals
in order to have them included in the glossary addendum.

Sincerely,
    Fabio, Maria Rita and Paolo.

LaTeX document follows.
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\subsection{History}

\entry{Definition}         

A {\em history} is the temporal representation of an ``object''
of the real world or of a database.
Depending on the object, we can have
{\em attribute histories, entity histories, relationship histories,
schema histories, transaction histories,\/} etc.

\entry{Alternative Names}   

Time sequence, time-series, temporal value, temporal evolution.

\entry{Discussion}

``History'' is a general concept, intended in the sense of
``train of events connected with a person or thing''.
Although it usually has to do with {\em past\/} events ($-$E5),
its use for the future 
--- as introduced by prophecies, science fiction, scientific forecasts ---
does not seem to present comprehension difficulties 
(there are much more problems with the adjective ``historical'').
Talking about future history, requires the same extension of meaning
as required by talking about future data.

In the realm of temporal databases, the concept of history is intended
to include multiple time dimensions as well as the data models (+R1).
Thus we can have valid-time histories, transaction-time histories, 
bitemporal histories, user-defined histories, etc.
However, multi-dimensional histories can be defined from mono-dimensional
ones (e.g. a bitemporal history can be seen as the 
transaction-time history of a valid-time history).

Formally or informally, the term ``history'' has been often used in 
many temporal database papers (+R4), also to explain other terms.
For instance, salary history, object history,
transaction history are all expressions used in this respect.

The alternative term ``temporal value'' is less general,
since it applies when ``history'' specializes into attribute history 
(value history).
Moreover, ``history'' is a slightly more general concept than
``time sequence'': differnet time sequences (with different
time granularities) could be extracted from the same history.
Therefore the definition of ``history'' does not prevent
defining ``time sequence''.

``History'' is also preferred over alternative names because
it allows a better definition of related terms.
Since it implies the idea of time, ``history'' does not require
futher qualifications as ``sequence'' or ``series'' do (+E2).
In particular, ``history'' well lends itself to be used
as modifier (+E1), even though ``time sequence''
is an alternative consolidated term ($-$E3,$-$E6).

``History'' is natural (+E8) and precise (+E9),
whereas ``temporal value'' may recall a temporal element
(e.g. timestamp value) and ``time sequence'' may recall
a sequence of temporal elements.

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\subsection{History-oriented}

\entry{Definition}         

A temporal DBMS is said to be {\em history-oriented\/} if:
\begin{enumerate}
%
\item It supports history unique identification
(e.g. via time-invariant keys, surrogates or OIDs);
%
\item The integrity of histories is inherent
in the model, in the sense that history-related integrity constraints
might be enforced and the language provides a mechanism
(history variables and quantification) for direct reference to
histories;
%
\item The DML allows easy manipulation of histories, in the sense
that the language provides for user-friendly history selection,
history retrieval and history modification primitives.
%
\end{enumerate}

\entry{Alternative Names}   
With temporal value integrity, grouped, object-oriented.

\entry{Discussion}
``History-oriented'' is preferred over ``with temporal value integrity''
since its meaning seems to be more direct.
Furthermore, in a more general perspective,
integrity constraints can be introduced
in a history-oriented model
(e.g. history uniqueness, entity history integrity,
referential history integrity).

``History-oriented'' is also preferred over ``grouped'' (+E7) in order
to avoid confusion with other kinds of grouping 
(e.g. defined terms ``[dynamic/static] valid time grouping'').

``History-oriented'' is not a synonim for ``object-oriented'',
even though a good temporal object-oriented model should also
be history-oriented. In general, object-orientation requires more
features that are inherited from snapshot O-O models (+E7).
For instance, also (attribute/tuple --- point/interval-stamped) 
relational models can be history-oriented, provided that 
suitable integrity constraints and algebraic operators are defined.

Once {\em history\/} has been defined, ``history-oriented'' is 
quite intuitive (+E8).

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\subsection{History equivalent}

\entry{Definition}         
Two objects are {\em history equivalent\/}
if they are equal for all $n$-dimensional
time boxes over which they are defined.
{\em History equivalence\/} is a binary relation
that can be applied to objects of any kind
(of the real world or of a database).

\entry{Alternative Names}   
Value equivalent, snapshot equivalent.

\entry{Discussion}
The ``value equivalence'' defined for tuples
could be extended to consider histories.
However, such an
extension would be rather inappropriate (+E1): value equivalence
concerns attribute values and completely disregards time,
whereas history equivalence implies a common evolution along
with time (implicitly assumes equality of timestamps prior
to compare data values).
The extension would violate the rationale of the introduction
of history-oriented models.

``History equivalent'' is a concept closer to ``snapshot equivalent'' ($-$E3)
rather than to ``value equivalent'' (+E5).
Anyway, ``history equivalent'' seems to be more general and intuitive (+E8).
An alternative definition could be: ``Two objects are {\em history equivalent\/}
if their {\em histories\/} are {\em shapshot equivalent.}

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