You can use nested table locators to improve performance when retrieving data.
Collection types do not map directly to a native type or structure in languages such as C++ and Java. An application using those languages must access the contents of a collection through Oracle database interfaces, such as OCI.
Generally, when the client accesses a nested table explicitly or implicitly (by fetching the containing object), the database returns the entire collection value to the client process. For performance reasons, a client may wish to delay or avoid retrieving the entire contents of the collection. Oracle database handles this case for you by using a locator instead of the actual nested table value. When you really access the contents of the collection, they are automatically transferred to the client.
A nested table locator is like a handle to the collection value. It attempts to preserve the value or copy semantics of the nested table by containing the database snapshot as of its time of retrieval. The snapshot helps the database retrieve the correct instantiation of the nested table value at a later time when the collection elements are fetched using the locator. The locator is scoped to a session and cannot be used across sessions. Because database snapshots are used, it is possible to get a
snapshot too old error if there is a high update rate on the nested table. Unlike a LOB locator, the nested table locator is truly a locator and cannot be used to modify the collection instance.
"Nested Table Locators" for more specific information