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In order to allow fast random access, an index structure may be used.
A file may have several indices on different search keys.
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.
Indices whose search key specifies an order different from the sequential
order of the file are called the secondary indices,
or nonclustering indices.
Mon Jul 13 13:28:03 PDT 1998