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000101_csj@iesd.auc.dk _Sun May 2 20:54:17 1993.msg
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Received: from iesd.auc.dk by optima.CS.Arizona.EDU (5.65c/15) via SMTP
id AA24394; Sun, 2 May 1993 11:56:18 MST
Received: from yellow.iesd.auc.dk by iesd.auc.dk with SMTP id AA05936
(5.65c8/IDA-1.5/MD for <tsql@cs.arizona.edu>); Sun, 2 May 1993 20:54:17 +0200
Date: Sun, 2 May 1993 20:54:17 +0200
From: "Christian S. Jensen" <csj@iesd.auc.dk>
Message-Id: <199305021854.AA05936@iesd.auc.dk>
To: tsql@cs.arizona.edu
Subject: Glossary entry for "coalesce."
GLOSSARY ENTRY FOR COALESCE.
Below is a proposal for a glossary entry for coalesce. The entry is
motivated by and partially based on a draft proposal by Nandlal Sarda.
Still, I accept the full responsibility for any deficiencies.
Best regards,
Christian S. Jensen
Aalborg University
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\begin{document}
\subsection{Coalesce}
\entry{Definition}
The {\em coalesce\/} operation takes as argument a set of
value-equivalent tuples and returns a single tuple which is snapshot
equivalent with the argument set of tuples.
\entry{Alternative Names}
Merging.
\entry{Discussion}
Coalesce is an example of a snapshot-equivalence preserving operation
which reduces the cardinality of a set of argument tuples.
The concept of coalescing has found widespread use in connection with
data models where tuples are associated with interval-valued
timestamps. In such models, two or more value-equivalent tuples with
consecutive or overlapping timestamps typically are required to be or
may be replaced by a single, value-equivalent tuple with an
interval-valued timestamp which is the union of the timestamps of the
original tuples.
There appears to be general consensus with respect to the name of this
concept (+E3). The name ``merging'' is occasionally used when
describing coalescing, but it has a less specific meaning and has not
been proposed as a substitute for ``coalescing'' ($-$E3, $-$E9).
\end{document}