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In order to allow fast random access, an index structure may be used.
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A file may have several indices on different search keys.
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If the file containing the records is sequentially ordered,
the index whose search key specifies the sequential order of
the file is the primary index, or clustering index.
Note: The search key of a primary index is usually the primary key,
but it is not necessarily so.
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Indices whose search key specifies an order different from the sequential
order of the file are called the secondary indices,
or nonclustering indices.
Osmar Zaiane
Mon Jul 13 13:28:03 PDT 1998