低HWM一下的块是确保已经被格式化过的。而低HWM至高HWM的块需要进行判断才能确定这个块是否是一个格式化过的块。
Concepts:
The low HWM is important in a full table scan. Because blocks below the HWM are formatted only when used, some blocks could be unformatted, as in Figure 12-25. For this reason, the database reads the bitmap block to obtain the location of the low HWM. The database reads all blocks up to the low HWM because they are known to be formatted, and then carefully reads only the formatted blocks between the low HWM and the HWM.
