Oracle Locally Managed Tablespace:
It is a locally managed tablespace that tracks all extent data in the tablespace itself by using bitmaps, resulting in the following benefits.
Key Benefits:
•It has Fast, concurrent space operations. Space allocations and deallocations modify locally managed resources (bitmaps stored in header files).
•This was a developed performance.
•Readable standby databases are enabled, because locally managed temporary tablespaces do not generate any undo or redo.
•It’s space allocation is simplified, because when the auto allocate clause is specified, the database automatically selects the appropriate extent size.
•The user reliance on the data dictionary decreased, because the required data is stored in file headers and bitmap blocks.
•Here, Coalescing free extents is not required for locally managed tablespaces.
All types of tablespaces, including the system tablespace, can be locally managed. The dbms_space_admin package offers maintenance procedures for locally managed tablespaces.
How to Create a Locally Managed TableSpace:
To Create a Locally Managed TableSpace by specifying local in the extent management clause of the create tablespace statement. You can have the database manage extents for you automatically with the auto allocate clause (the default), or you can specify that the tablespace is managed with uniform extents of a specific size (uniform). Auto allocate is also one of the best choices if it is not important for you to have a lot of control over space allocation and deallocation, because it simplifies tablespace management. Some of the space may be wasted with this setting, but the benefit of having an oracle database manage your space most likely outweighs this drawback.
When you do not explicitly specify the type of extent management, oracle database determines extent management as below:
•If the create tablespace statement omits the default storage clause, then the database creates a locally managed auto allocate tablespace.
•If the create tablespace statement includes a default storage clause, then the database considers like this: If you specified the minimum extent clause, the database evaluates whether the values of minimum extent, initial, and next are equal and the value of pctincrease is 0. If so, the database creates a locally managed uniform tablespace with extent size = initial. If the minimum extent, initial, and next parameters are not equal, or if pctincrease is not 0, the database refuses any extent storage parameters you may specify and creates a locally managed, auto allocate tablespace.
•If you did not specify minimum extent clause, the database evaluates only whether the storage values of initial and next are equal and pctincrease is 0. If so, the tablespace is locally managed and uniform. Otherwise, the tablespace is locally managed and auto allocated.
The below statement creates a locally managed tablespace named lmtbsb and specifies auto allocate:
Sql> create tablespace lmtbsb datafile ‘/u02/oracle/data/lmtbsb01.dbf’ size 50m
Extent management local autoallocate;
This Autoallocate causes the tablespace to be system managed with a minimum extent size of 64k.
The alternative to autoallocate is uniform. Then it specifies that the tablespace is managed with extents of uniform size. With this you can specify that size in the size clause of the uniform. If you leave size, then the default size is 1m.
Sql> create tablespace lmtbsb datafile ‘/u02/oracle/data/lmtbsb01.dbf’ size 50m
Extent management local uniform size 128k;
With the help of this, you cannot specify the default storage clause, minimum extent, or temporary when you explicitly specify extent management local. If you want to create a temporary locally managed tablespace, use the create temporary tablespace statement.
Specifying segment space management in locally managed tablespaces:
In a locally managed tablespace, there are two processes that an oracle database can use to manage segment space: automatic and manual. Manual segment space management uses linked lists said to be “freelists” to manage free space in the segment, while automatic segment space management uses bitmaps. This space management is the more efficient process, and is the default for all new permanent, locally managed tablespaces.
Automatic segment space management delivers better space utilization than manual segment space management. It is also self-tuning, in that it scales with an increasing number of clients or examples. In addition, for many standard workloads, application performance with automatic segment space management is better than the performance of a well-tuned app using manual segment space management.
The below statement creates tablespace lmtbsb with automatic segment space management:
Sql> create tablespace lmtbsb datafile ‘/u02/oracle/data/lmtbsb01.dbf’ size 50m
Extent management local
Segment space management auto;
This segment space management manual clause disables automatic segment space management.
The segment space management that you specify at tablespace creation time applies to all segments subsequently created in the tablespace. With this you cannot change the segment space management mode of a tablespace.
•If you set extent management to local uniform, then you must ensure that each extent contains at least 5 database blocks.
•If you set extent management to local auto allocate, and if the database block size is 16k or greater, then oracle manages segment space by creating extents with a minimum size of 5 blocks rounded up to 64k.
Conclusion:
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