Oracle Unleashed- P7: When I first started using Oracle many years ago, it was possible to know the database and the tools available. With the rash of recent releases of different options for the database and the spate of new tools, only people who wear their underpants over their trousers will be able to know everything there is to know about the Oracle products. | Segments and Extents When a segment such as a table is created the storage parameters should be explicitly set for the segment. If no storage parameters are in effect the segment uses the default storage parameters which you can specify by each tablespace so that objects created in different tablespaces use different default storage parameters. For each user a tablespace can be identified as a default tablespace. Therefore if the tablespace clause is omitted from a CREATE statement the user will use the default tablespace defined for him. Assuming that all the default settings are used when a segment is created it will occupy a contiguous set of Oracle blocks in the database files when the CREATE statement succeeds. The segment has those Oracle blocks allocated to it and waits for data to occupy them. The first set of contiguous Oracle blocks allocated to the segment is called the segment s initial extent. Suppose that the segment is a table. As rows are inserted into the table they use the storage in the Oracle blocks allocated to the table in the table s initial extent until no more rows can be inserted into those Oracle blocks. After all the Oracle blocks of the first extent have no more room for any rows the table automatically uses more Oracle blocks in the tablespace as further rows are inserted into the table. That is it uses another extent of Oracle blocks. This happens invisibly to the user who is inserting rows although a slight delay might occur on some systems as another extent is allocated to the segment. The process repeats until the table reaches the maximum number of extents that can be allocated or until no more free space is available to allocate to the table. Additional extents allocated to the segment after the first extent are called next extents for the object. __The maximum number of extents that can be allocated depends on the size of the Oracle block. For a 2K Oracle block the maximum number of extents that can be allocated is 121. .
