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High Water Mark
All blocks that at some point contained data
Notes:
A full table scan reads all blocks up to the “high water mark”. The high water mark is a somewhat imaginary line that represents the demarcation between those blocks within a segment that contain or once contained data, and those blocks that never contained data.
As the example above demonstrates, a DELETE statement does not reset (move) the high water mark. The full table scan after the delete reads the same number of blocks as the scan before the delete (1505).
See the script high_water_mark.sql for a working demonstration of this concept.
Next, let’s discuss solutions…