Difference between extent management and segment space management

Question

Hello,
Could you tell me the difference between extent management and segment space management interms of datablocks?

Answer 1

After 10g, default tablespace is locally managed and with this creation, your segment's spaces are managed "automatically".
Automatic segment space management is onlyt avaliable to locally managed tablespaces.
The logic of the automatic segment space management is;
For instance, we have a rectangle cube which is totally empty (1 extent - 8 segments - 64 block in default). If we enter a new data to our table, data is stored inside this cube with it's information. (default 8k). If our data must be splitted, check row chaining for further information.
At this point, oracle decides which block is filled with the entered data. It automatically fills the emty blocks. You can use this only if your tablespace is locally managed and your segment space managemet is auto.
On the other hand,
If we create dictionary managed tablespace,
Data is stored in "dictionary" (tables in SYS schema). This is a bit slower than locally managed tablespaces. So filling the extents, segments and blocks will be slow too. You need to specify the PCTUSED, FREELISTS, FREELISTS GROUPS for the storage of the data. If segment space managent is auto, you don't need to define those values.
In summary, locally managed tablespaces with automatic segment space management is Oracle's maybe the most important feature. I am not using dictionary managed tablespaces and i advice don't use it either. In the end, this is an upgrade of using segment spaces much more faster, easier and reliable, trustable.
PS: You will need to check "High Water Mark" for automatic segment space management.