High Water Mark

It is a marker that shows you the point when you last inserted the data in the table. Let us simplify it with an example. Suppose you created a new table initially with no value. The high water mark will be at initial position say at 0. You made an insertion. The HWM will rise by one and will reach at position 1. Similarly as you will make more and more insertions the HWM will also rise; suppose you reached at a position where the value of HWM was 100. Now you started deleting values from the table. The value of HWM will not fall with the deletion of values from the table. So you can easily say that HWM shows the highest level of insertion you have ever done in that particular table.

Every time you give the command of select to fetch values from a table the oracle searches the table to HWM. It does not matter whether your last value is stored on position 10 or 90; if your HWM is 100 then oracle will search up to 100 positions. Normally for deleting values from the table you use the command of delete. This does not affect your HWM even if you have deleted all the values from the table. To bring the HWM to the initial position you can use the command:

SQL>truncate table table_name;

The high water mark is only meaningful in the table segment.