RelationalDBDesign 




Table Space Management   «Prev  Next»
Lesson 2 Overview of tablespace management
Objective Describe the purpose of tablespace management.

Extended and restricted rowid

Because you have either taken other courses about Oracle administration or have some on-the-job experience with it, you already understand how tablespaces fit into the Oracle architecture.
This lesson discusses two key concepts in tablespace management. The contents of the
  1. rowid and
  2. the methods of tablespace extent management.
The rowid contains a record location. In legacy Oracle databases, the rowid was formatted in an absolute address that was tied to the database. In Oracle, this format, called the restricted rowid, is kept only for backward compatibility.
An extended rowid is the new Oracle format for rowid. All rows created in an Oracle database use the extended rowid. The extended rowid is a tablespace-relative address for a record, which makes the rowid portable across databases.

The primary purpose for revising the format of the rowid is to enable transportable tablespaces. Transportable tablespaces are covered later in this module.


Logical rowid
A new type of rowid called the logical rowid is used to track the location of rows in index-organized tables.
Rows in an index-organized table can move to a different physical location when new rows are inserted or old rows are deleted. Therefore, a traditional rowid (containing the physical location of the record) does not serve the purpose intended for a rowid (a permanent and direct locator of the row). The logical rowid solves this problem by storing information about location that includes primary key information rather than physical block information.
To use this kind of rowid in your PL/SQL code, use the UROWID data type.

Changes in Tablespace extent Management

The biggest change in the sizing of a tablespace is the capability to choose between two methods of tablespace sizing and extent management:
  1. The traditional method, called dictionary managed, which uses the database dictionary to track data blocks and allocate extents. This is the default method.
  2. The new method, called locally managed, which uses the tablespace itself to track data blocks and allocate extents. This method can be more efficient because it reduces calls to the database dictionary.
The new method is featured in the next lesson.
In the next lesson you will find out all about locally managed tablespaces and how to create them.