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High water mark [HWM] | ||
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.
Links
Diagnostic event 10081.
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