High Water Mark in Oracle
High water mark is the maximum amount of database blocks used so far by a segment. This mark cannot be reset by delete operations.
Delete Table operation won’t reset HWM.
TRUNCATE will reset HWM.
The high water mark level is just a line separate the used blocks and free blocks.
The blocks above the HWM level is free blocks, they are ready to use.
The blocks below the HWM level is used blocks, they are already used.
I hope now you understand well. see the below example.
for example, if you delete some huge records from the oracle dba database, that data will delete but the blocks are not ready to used, because that blocks are still below HWM level, so delete command never reset the HWM level,
At the same time you truncate the date, then the data will delete and that use