Tablespace Management (Continued)

Managing Tablespace means to tell that what its extent management is. We know that inside a Tablespace there are many segments which in turn consist of many extents. There are two ways to manage a Tablespace.

1. Dictionary Managed
2. Locally Managed

Suppose there is a tablespace named as T1, inside T1 there is a segment named S1, and inside S1 there are four extents named as E1, E2, E3, and E4. Here we need to remember is that which extents are full and which are empty. For example for extent E1 entry will be:

T1-S1-E1: empty/full.

And similar for all other extents in that tablespace and in other tablespaces in the database. If you are managing your tablespaces using dictionary managed than all this information will be stored in the data dictionary. And data dictionary is consulted every time a tablespace is used. For storing this information in the data dictionary there are two base tables. One is UET$ and the other is FET$. The first one stands for used extent table and stores the information of all the extents that are used or full. The second one stands for free extent table and stores the information of all extents that are free. When an extent is free it will be in FET$ table and after it is filled its entry is removed from FET$ and is stored in the UET$ table.

The other way is to manage the tablespace locally without using the data dictionary. The extent information is held in the headers of data files in the form of bit maps. The bit map will contain all the information about the extents that whether they are free or full. For example a bit map will show 1 for the extent that is full and 0 for the extent that is free. This is a more preferred approach because it removes the overhead of consulting data dictionary every time you need to use the tablespace.

Both approaches are used. But as mentioned that locally managed is much better option and oracle highly recommends it. Locally managed was introduced in 8i. You can also manage the system tablespace. But for 8i release 2 and early versions you can’t manage it using locally managed. In later versions you can do this. But remember that if have managed the system tablespace using locally managed method than you will not be able to manage other tablespaces using dictionary managed method. You had to use locally managed through out the system. This is not the case with dictionary managed.