ORACLE HWM 和 low HWM

今天在学习的过程中看到了LOW HWM，印象中以前在什么地方看到过可是死活想不起来，去百度了半天都没找到相关的信息。无奈之下还是GOOGLE了一下，找到一篇asktom上的帖子。记录在这里，顺便和大家分享一下。
High-water Mark
This is a term used with table segments stored in the database. If you envision a table, for example, as a 'flat' structure or as a series of blocks laid one after the other in a line from left to right, the high-water mark (HWM) would be the rightmost block that ever contained data, as illustrated in Figure 10-1.
+---- high water mark of newly created table
|
V
+--------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
high water mark after inserting 10,000 rows
|
v
+--------------------------------------------------------+
|x |x |x |x |x |x |x |x |x |x |x |x | | | | | | | |
|x |x |x |x |x |x |x |x |x |x |x |x | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
high water mark after inserting 10,000 rows
|
v
+--------------------------------------------------------+
|x |x |x |x |x |x |x | | | | | | | | | | | | |
|x |x |x |x |x |x |x | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 10-1. Depiction of an HWM
Figure 10-1 shows that the HWM starts at the first block of a newly created table. As data is placed into the table over time and more blocks get used, the HWM rises. If we delete some (or even all) of the rows in the table, we might have many blocks that no longer contain data, but they are still under the HWM, and they will remain under the HWM until the object is rebuilt, truncated, or shrunk (shrinki