Every data block in an ASSM segment is in one of the following states:
- Above the HWM
These blocks are unformatted and have never been used.
- Below the HWM
These blocks are in one of the following states:
- Allocated, but currently unformatted and unused
- Formatted and contain data
- Formatted and empty because the data was deleted
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.
How to View High Water Mark – Step-by-Step Instructions (Doc ID 262353.1)