RE: what are leaf blocks and table high water marks?

A Leaf Block is an index block. In a B*tree index, index blocks are either branch blocks (the "top" blocks within the B*tree index), or leaf blocks
(the "lower" level index blocks). Branch blocks hold index data that point
to lower level index blocks. Leaf blocks hold every indexed data value and a corresponding ROWID used to locate the actual row of data.

See:

http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96524/c11schem .htm#3353

A tables High Water mark is a mark that is set against a table, pointing to the highest amount of data that has been inserted in to that table at any one time. Say you inserted 1,000,000 rows in to the table, then delete 200,000, the HWM would point at 1,000,000 rows. It is used as a boundary for space management, and can also be used by the optimiser to make poor choices under certain circumstances.. ;) You can't release space below the high water mark, even if there is no data below that high water mark (an empty table). The only way to do this is via the TRUNCATE command.

See:

http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96521/schema.h tm#6870

HTH Mark

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-----Original Message-----
mldelosreyes_at_ucpb.com
Sent: 15 October 2003 10:39
To: Multiple recipients of list ORACLE-L

can anyone explain what are leaf blocks and table high water marks? thanks.

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