Nonfragmented tables contain a hidden column called rowid. Its integer value defines the physical location of a row.
3To include a rowid column in a fragmented 3table, you must explicitly request it by using the WITH ROWIDS clause in CREATE 3TABLE (or ADD ROWIDS in ALTER TABLE, or WITH ROWIDS in ALTER FRAGMENT INIT). 3 The rowid in a row of a fragmented table does not identify 3a physical location for the row in the same way that a rowid in a non-fragmented table does.
When you use the WITH ROWIDS option to add a new rowid column to a fragmented table, the database server assigns a unique rowid number to each row and creates an index that it can use to find the physical location of the row. Performance using this access method is comparable to using a SERIAL or SERIAL8 column. The rowid value of a row cannot be updated, but remains stable during the existence of the row. Each row requires an additional four bytes to store the rowid column after you specify the WITH ROWIDS option.