each segment starts out with 140 Chapter 3 Database Storage and Schema Objects, 64KB extents for the first 16 extents, and then extents increase in size to 1MB for the next 63 extents. The size then increases to 8MB for the next 120 extents, then 64MB, and so on as the segment grows.
*先分配16个64K的extent，
0-15 extents 每个大小是64K 合计大小 1M
16－79 extents 每个大小是1M 合计大小 63M --以上两项大小合计 64M
80－199 extents 每个大小是8M 合计大小 960M --以上三项大小合计 1024M＝1G
200－？？ extents 每个大小是64M
*/
注：每次分配的Extent的大小，可参见DBA_Extents字典表
