Role of Oracle extents

Each segment is composed of extents, which are groups of contiguous data blocks.
Oracle manages the use of extents, allocating a new one when one is full.

An extent is a logical unit of database storage space allocation made up of a number of contiguous data blocks. One or more extents in turn make up a segment. When the existing space in a segment is completely used, Oracle allocates a new extent for the segment.

When you create a table, Oracle allocates to the table's data segment an initial extent^[1] of a specified number of data blocks. Although no rows have been inserted yet, the Oracle data blocks that correspond to the initial extent are reserved for that table's rows. If the data blocks of a segment's initial extent become full and more space is required to hold new data, Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previously allocated extent in that segment. For maintenance purposes, the header block of each segment contains a directory of the extents in that segment.

Storage parameters expressed in terms of extents define every segment. Storage parameters apply to all types of segments and control how Oracle allocates free database space for a given segment.
For example, you can

1. determine how much space is initially reserved for a table's data segment or
2. you can limit the number of extents the table can allocate by specifying the storage parameters of a table in the STORAGE clause of the CREATE TABLE statement.

If you do not specify a table's storage parameters, then it uses the default storage parameters of the tablespace.
You can have dictionary managed tablespaces, which rely on data dictionary tables to track space utilization, or locally managed tablespaces, which use bitmaps to track used and free space. Because of the better performance and easier manageability of locally managed tablespaces, the default for non-SYSTEM permanent tablespaces is locally managed whenever the type of extent management is not explicitly specified. A tablespace that manages its extents locally can have either

1. uniform extent sizes or
2. variable extent sizes

which are determined automatically by the system. When you create the tablespace, the UNIFORM or AUTOALLOCATE (system-managed) clause specifies the type of allocation.

The storage parameters INITIAL, NEXT, PCTINCREASE, and MINEXTENTS cannot be specified at the tablespace level for locally managed tablespaces. They can, however, be specified at the segment level. In this case, INITIAL, NEXT, PCTINCREASE, and MINEXTENTS are used together to compute the initial size of the segment. After the segment size is computed, internal algorithms determine the size of each extent.

You can specify the size of an extent for a database object with the CREATE command for the object, or alter the size of subsequent extents with the ALTER command for the object.
When you first create a database table, Oracle allocates an initial extent for the segment. If the data in the table gets too large to fit in the initial extent, Oracle allocates a new one. The following series of diagrams illustrate how this happens.

The incremental extent can be the same size as the initial extent, or you can indicate that each subsequent extent must be a certain percentage larger than the previous extent. For example, you could indicate that each subsequent extent must be 50% larger than the previous one. You can also specify the minimum and maximum number of extents. The minimum number of extents controls how many extents are initially allocated when you create the object. If you specify a maximum number of extents, it has the effect of limiting the size of the table.
The next lesson shows how to set extent parameters for database objects.

Click the Quiz link below to answer a few questions about space management.
Space Management - Quiz

[1]initial extent:The INITIAL size is the size of the initial extent allocated when the object is created.