Extents (Continued)

A segment has more than one extent in it. Extents are the basic allocation units. Suppose you created a tablespace with the following command:

SQL>create tablespace test datafile ‘c: \oracle\oradata\dbase\test01.dbf’ size 10M extent management local uniform size 512K.

Here the second size mentioned 512K is the size of extents created within each segment. By uniform we mean that the size will be same for all extents of all segments. You can override this behavior for a segment by using the storage clause e.g.

SQL>create table t1 (t_no number)

storage

(initial 1M

next 1M

);

Now all the extents created for the table T1 will be of size 1M instead of default 512K. By initial we mean the size of first extent and next are used for the every next extent.

As we know that the extents are the basic allocation units. It means that the size is allocated to tablespace according to the size of extent. When you turn the autoextend on then for example a segment needs more space. In the above example the size of more space given at once to the segment will be 1M i.e. the size of the extent.