Subject: Glossary entry for "coalesce."

From: "Christian S. Jensen" <[email-masked]>
Date: Sun, 2 May 1993 20:54:17 +0200

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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\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).

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