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High water mark [HWM]

The high water mark is divides a segment into used blocks free blocks
Blocks below the high water mark (used blocks) have at least once contained data. This data might have been deleted.
Since Oracle knows that blocks beyond the high water mark don't have data, it only reads blocks up to the high water mark in a full table scan.
Oracle keeps track of the high water mark for a segment in the segment header.

Moving the high water mark

In normal DB operations, the high water mark only moves upwards, not downwards. The exceptions being the truncate.
If there is a lot of free space below the high water mark, one might consider to use alter table move statements. See On shrinking table sizes.

Initial position

The initial position of the high water mark is extent 0 block 0 for tables and extent 0 block 1 for indexes.