When the data is stored in a relational
table, the unique identifier is usually the primary key. This implementation
means that you can use object-oriented programming techniques without converting
existing relational tables to object-relational tables. The tradeoff when using this
approach is that performance may be less than optimal, since the data representing
attributes for an object may reside in several different tables. Hence, it may make
sense to convert the relational tables to object tables in the future.
Objects that share the same attributes and methods are said to be in the same datatype
or class. For example, internal and external purchase orders can be in the same class as
purchase orders. Collection types model a number of objects of the same datatype as
varying arrays (VARRAYs) if the collection of objects is bounded and ordered or as
nested tables if the collection is unbounded and unordered. If a collection has fewer
than 4,000 bytes, it is stored as part of the database table; if it is larger, it is stored as a
Binary Large Object (BLOB) in a segment separate from the table that is considered
???out-of-line??? storage.
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