The System Global Area in a Oracle database is the area in memory to facilitates the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database. The structure is Database buffers, Dictionary cache, Redo Log Buffer and Shared SQL pool (ver 7.0 only) area.
The data dictionary cache is stored in an area in SGA called the Shared SQL Pool. This will allow sharing of parsed SQL statements among concurrent users.
It is area in memory that is used by a Single Oracle User Process.
The space used transaction entries and a deleted record does not become free immediately after completion due to delayed cleanout.
Trailing nulls and length bytes are not stored. Inserts of, updates to and deletes of rows as well as columns larger than a single datablock, can cause fragmentation and chained row pieces.
Full back - Complete database
Incremental - Only affected tables from last incremental date/full backup date.
Cumulative backup - Only affected table from the last cumulative date/full backup date.