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Extents in Data Dictionary

Oracle Tablespace and Storage Manager

  1. You have already selected the Storage Manager menu choice from the DBA Management Pack group of the Oracle Enterprise Manager program group. Click Storage Manager.
  2. You should log in as an existing user--in this case, use the user name of sys and a password of sys to see a variety of tablespaces. Once you have entered the username and password, click OK to bring up Storage Manager.
  3. When Oracle Storage Manager comes up, click Tablespaces in the list box at left. (Note: The display has been resized from the default display for better understanding in this simulation.)
  4. The size of each tablespace is displayed in the right hand frame, along with the amount of space it used. If you want to explore the tablespaces further, you can select a particular tablespace. In this case, click the USER_DATA tablespace.
  5. Once you have selected a particular tablespace, you can click the Show Dependencies toolbar button at far left, the third button from the top.
  6. The Dependencies dialog comes up, by default, with the Dependencies tab showing. You can click the plus sign to the left of the Datafile folder to display the datafiles associated with the tablespace.
  7. You can get more information about the tablespace by clicking on the Dependents tab of the Dependencies window.
  8. This tab of the Dependencies window shows the logical entities that reside in the USER_DATA tablespace. You can show the tables that reside in this tablespace by expanding the Tables portion of the USER_DATA tree.
  9. From here, you can drill down into further dependencies by expanding a particular table, which shows you the dependencies of other database objects on the table. This is the end of the simulation.

Extents and Segments

From a physical point of view, a datafile is stored as operating system blocks. From a logical point of view, datafiles have three intermediate organizational levels: 1) data blocks, 2) extents, and 3) segments. An extent is a set of data blocks that are contiguous within an Oracle datafile. A segment is an object that takes up space in an Oracle database, such as a table or an index that is composed of one or more extents.
When Oracle updates data, it first attempts to update the data in the same data block. If there is not enough room in the data block for the new information, Oracle will write the data to a new data block that could be in a different extent. This discussion is especially important if you are running an older release of Oracle. Oracle Database 10g added a Segment Advisor that greatly simplifies reclaiming unused space in current database versions.