High Water Mark

Every data block in an ASSM segment is in one of the following states:

  • Above the HWM

    These blocks are unformatted and have never been used.

  • Below the HWM

    These blocks are in one of the following states:

    • Allocated, but currently unformatted and unused
    • Formatted and contain data
    • Formatted and empty because the data was deleted

The low HWM is important in a full table scan. Because blocks below the HWM are formatted only when used, some blocks could be unformatted, as in Figure 12-25. For this reason, the database reads the bitmap block to obtain the location of the low HWM. The database reads all blocks up to the low HWM because they are known to be formatted, and then carefully reads only the formatted blocks between the low HWM and the HWM.

Reference:

https://docs.oracle.com/cd/E11882_01/server.112/e40540/logical.htm#CNCPT89022
How to View High Water Mark – Step-by-Step Instructions (Doc ID 262353.1)

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s