將tablespace 從data dictionary mgt. --> local extent mgt(bitmap)

前言.
從8i 舊版本，SYSTEM tablespace的資訊檢索為dictionary based tablespace. 此存取方式有點類似B tree的方法. 但如果從8i 升到9i以上，可執行指令將其升為bitmap based tablespace(local managed) , 此資料型態讓檢索只需要查詢一次，故效率較好(如果為bitmap , 效率為Log N )

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Oracle 10g administration guide
b14231.pdf  (p.226)
Oracle 9i administration guide
a96521.pdf  (p.354)
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Migrating from a Dictionary-Managed to a Locally Managed Tablespace

Migrating the SYSTEM Tablespace to a Locally Managed Tablespace
SQL> EXECUTE DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL('SYSTEM');
Before performing the migration the following conditions must be met:
■ The database has a default temporary tablespace that is not SYSTEM.
■ There are no rollback segments in the dictionary-managed tablespace.
■ There is at least one online rollback segment in a locally managed tablespace, or if
using automatic undo management, an undo tablespace is online.
■ All tablespaces other than the tablespace containing the undo space (that is, the
tablespace containing the rollback segment or the undo tablespace) are in
read-only mode.
Transporting Tablespaces Between Databases
Managing Tablespaces 8-25
■ The system is in restricted mode.
■ There is a cold backup of the database.

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所有非system 的tablespace 須先轉為local mgt. 最後再將system 轉為local mgt.
否則在system 轉為local  mgt.後，其他dictionary mgt. tablespace 永遠只能停在read only mode.

Note: ''After the SYSTEM tablespace is migrated to locally managed, any dictionary-managed tablespaces in the database cannot be made read/write''. If you want to be able to use the dictionary-managed tablespaces in read/write mode, then Oracle recommends that you first migrate these tablespaces to locally managed before migrating the SYSTEM tablespace.