Data and index segments are stored in Oracle Database tablespaces. Tablespaces come in two varieties: both dictionary-managed and locally managed tablespaces.
Locally Managed Tablespaces
A locally overseen tablespace deals with its free space utilizing a bitmap that is put away in the tablespace header. This indicates that the allocation and deallocation of tablespace space is handled by the database itself. There are a number of benefits to local management of tablespaces, including:
1. enhanced efficiency: There is less overhead associated with managing the tablespace as a result of the database's improved capacity to allocate space within the tablespace.
2. Improved dependability: Since the database is in charge of allocating space, local management of tablespaces makes it less likely that errors will occur.
3. Better utilization of space: Because the database is able to allocate space more precisely, local management of tablespaces can result in improved space utilization.
Dictionary Managed Tablespaces (DMT):
A dictionary-managed tablespace, on the other hand, manages its free space by utilizing the data dictionary. The database makes use of the information contained in the data dictionary to manage the allocation and deallocation of space within the tablespace. Because the database must perform more operations to manage the tablespace, dictionary managed tablespaces can be slower and less efficient than locally managed tablespaces.
Oracle use the data dictionary (tables in the SYS schema) to track allocated and free extents for tablespaces that is in "dictionary managed" mode. Free space is recorded in the SYS.FET$ table, and used space in the SYS.UET$ table.
In conclusion, When Oracle allocates space to a segment (like a table or index), a group of contiguous free blocks, called an extent, is added to the segment. Metadata regarding extent allocation and unallocated extents are either stored in the data dictionary, or in the tablespace itself. Tablespaces that record extent allocation in the dictionary, are called dictionary managed tablespaces, and tablespaces that record extent allocation in the tablespace header, are called locally managed tablespaces.dictionary-managed tablespaces are generally regarded as inferior to locally managed tablespaces due to their superior performance, dependability, and capacity utilization. However, dictionary-managed tablespaces may be required in some situations due to compatibility issues with older Oracle versions or particular third-party software.
