The following steps explain how space within a block is managed for a data segment with
PCTFREE=20 and PCTUSED=40:
1. Rows are inserted into the block until the free space in the block is equal to or less
than 20%. The block is no longer available for inserts when rows occupy 80% (100 –
PCTFREE) or more of the available data space in the block.
当block空间小于100-PCTFREE，可以插入数据；否则就不能往该block插入数据。
2. The remaining 20% can be used when the size of a row increases. For example, a
column that was originally NULL is updated to be assigned a value. Thus block
utilization may be in excess of 80% as a result of updates.
而剩下的PCTFREE的数据用于已存数据的扩展（例如null数据-》value）。
3. If rows are deleted in the block or if rows decrease in size as a result of updates,
block utilization may fall below 80%. However, a block is not used for inserts until
the utilization falls below PCTUSED, which in this example, is 40%.
当数据删除及decrease操作引起block使用空间小于（100 –PCTFREE）时，该block不一定能插入数据，除非使用空间小于PCTUSED。
4. When the utilization falls below PCTUSED, the block is available for inserts. As
rows are inserted into the block, the utilization of the block increases and the cycle
repeats starting with step 1.
当block使用空间小于PCTUSED，block可以插入数据。