Index-only tables
In some cases, all the data that would normally be stored in a table can be stored in an index, and the table is not necessary. An index-only table, available starting with Oracle8, keeps the data sorted according to the primary key column. There are some limitations to this type of object:
Since the data is not stored in a table, there are no ROWIDs available.
A primary key must be defined for the table.
No additional indexes can be created; only the primary key may be indexed.
An index-only table is created by using the ORGANIZATION INDEX clause of the CREATE TABLE command.
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