Data Blocks, Extents, Segments

Block : Its the smallest unit of storage. For a database the block size is fixed,

however in 10g we could have multiple block sizes, default bloc size is 8K, the setting is

configured in initialization file i.e init.ora

Use the following sqlplus query to get the block size of the database

show parameter block_size

Extents : Grouping of specific number of contiguous data blocks.

Segments : Collection of extents, segment are of specific object types

table segment, index segment, cluster segment etc

Now practically when you create a new object a new segment is going to be

created for it with specified number of extents (default 1), now when the extents

get full oracle creates a new extent for the segment which may not be in contiguous

with the earlier extent

Lets check the dba_segments view

Following are some of the important columns in the view

Lets check the dba_extents view, important ones listed below