Subject: More glossary terms

From: "Michael Soo" <soo>
Date: Tue, 24 Nov 1992 16:08:25 MST

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\begin{document}

\subsection{Beginning}

\entry{Definition}         

The time-line supported by any temporal DBMS is, by necessity, finite
and therefore has a smallest and largest representable chronon.
The distinguished value {\em beginning\/} is a special valid-time event
preceding the smallest chronon on the valid-time line.
Beginning has no transaction-time semantics.

\entry{Alternative Names}   

Start, begin, commencement, origin, negative infinity.

\entry{Discussion}

Beginning has the advantage of being intuitive (+E8), and does
not have conflicting meanings (+E5).  

``Begin'' appears to be more straight-forward (+E8) but suffers from
conflicting meanings since it is a common programming 
language keyword (-E5).

``Start,'' ``commencement,'' and ``origin''
are awkward to use, e.g., ``Start precedes the event,'' 
``Commencement precedes the event,'' and 
``Origin precedes the event.'' (-E8).  
Furthermore, choosing start would require us to choose ``end'' for
the opposite concept, and end is a common programming language keyword
(-E5).  Origin also has a conflicting meaning relative to calendars (-E5).

Lastly, ``negative infinity'' is longer (-E2) and 
slightly misleading since it implies that time 
is infinite (-E9).  This may or may not be true depending on 
theories about the creation of the universe.
Also, negative infinity has a well-established mathematical meaning (-E5).

\subsection{Forever}

\entry{Definition}         

The distinguished value {\em forever\/} is a special valid-time event
following the largest chronon on the valid-time line.
Forever has no transaction-time semantics.

\entry{Alternative Names}   

Infinity, positive infinity.

\entry{Discussion}

Forever has the advantage of being intuitive (+E8) and does
not have conflicting meanings (+E5).  

``Infinity'' and ``positive infinity'' both appear to be more straightforward
but have conflicting mathematical meanings (-E5).  Furthermore,
positive infinity is longer and would require us to choose
``negative infinity'' for its opposite (-E2).

\subsection{Initiation}

The distinguished value {\em initiation\/} denotes the transaction-time
when the database was created, i.e., the chronon during which the 
first update to the database occurred.  Initiation has no valid-time
semantics.

\entry{Alternative Names}   

Start, begin, commencement, origin, negative infinity, beginning.

\entry{Discussion}

The arguments against ``start,'' ``begin,'' ``commencement,'' ``origin,'' 
and ``negative infinity'' are as in the discussion of beginning.

Initiation is preferred over beginning since transaction-time
is distinct from valid-time.  Using different terms for the 
two concepts avoids conflicting meanings (+E5).

\subsection{Fixed Span}

\entry{Definition}

The duration of a span is either context-dependent or context-independent.
A {\em fixed span\/} has a context-independent duration.
For example, the span {\tt one hour} has a 
duration of 60 minutes and is therefore a fixed span.

\entry{Alternative Names}

Constant span.

\entry{Discussion}

Fixed span is short (+E2), precise (+E9), and has no conflicting
meanings (+E5).

``Constant'' appears more precise (+E8) and intuitive (+E9), but
it is also used as a keyword in several programming languages
(-E5).

\subsection{Variable Span}

\entry{Definition}

A span that is not fixed is {\em variable\/}---the value of the span
is dependent on the context in which it appears.
For example, the span {\tt one month} represents a duration
of between twenty-eight and thirty-one days depending on the
context in which it is used.

\entry{Alternative Names}

Moving span.

\entry{Discussion}

Variable span is intuitive (+E9), and precise (+E9).  

``Moving span'' is unintuitive (-E9) and has informal spatial connotations
(-E5).

\end{document}