Permanently Removing Tables
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As noted previously, if you want to permanently and immediately remove a table, without moving it to the Recycle Bin, you must use the DROP TABLE table_name PURGE command: SQL> DROP TABLE persons PURGE; Table dropped. SQL>
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The new PURGE clause comes in especially handy when you want to drop a sensitive table and don t want it to appear in the Recycle Bin for security reasons.
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You can also use the PURGE TABLE or the PURGE INDEX command to permanently erase a previously dropped table or index from the Recycle Bin: SQL> PURGE TABLE persons Table purged. SQL>
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CHAPTER 16 DATABASE RECOVERY
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Alternatively, you can use the system-generated name: SQL> PURGE TABLE "BIN$Q1qZGCCMRsScbbRn9ivwfA==$0" Table purged. SQL> If you have several tables of the same original name in the Recycle Bin, the PURGE command will drop the first table that you originally dropped.
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Once you remove an object from the Recycle Bin with the PURGE command, or when you drop an object with the PURGE option, you can t apply the Flashback Drop feature to retrieve those objects (or their dependent objects) the purged objects are gone forever!
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You can also use the PURGE TABLESPACE command to remove all objects from the Recycle Bin that are part of that tablespace, as shown here: SQL> PURGE TABLESPACE users; The following command will remove all objects of a single user, scott (along with any dependent objects that live in other tablespaces) from the tablespace users: SQL> PURGE TABLESPACE users USER scott; To permanently remove all objects from a tablespace, without them moving to the Recycle Bin, you can use the DROP TABLESPACE . . . INCLUDING CONTENTS command. In addition, any objects belonging to the tablespace that are currently in the Recycle Bin are immediately purged. The DROP TABLESPACE command by itself, without the INCLUDING CONTENTS clause, will fail unless the tablespace is empty. If you wish to permanently remove all of your objects currently in the Recycle Bin, you can use the PURGE RECYCLEBIN command (or PURGE USER_RECYCLEBIN). These will simply remove any objects belonging to the user issuing the command. In order to empty the entire Recycle Bin of all objects, regardless of ownership, you can use PURGE DBA_RECYCLEBIN. However, for obvious reasons, you need the SYSDBA privilege to issue this command.
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The DROP USER . . . CASCADE command will instruct Oracle to drop the user and all objects owned by the user from the database and will automatically purge any objects in the Recycle Bin that belong to that user.
Finally, remember that Oracle may automatically purge objects from the Recycle Bin if it experiences space pressure. It will start with the oldest objects.
To retrieve a table using the FLASHBACK TABLE table_name TO BEFORE DROP command, you must either be the owner or have the drop privileges (DROP TABLE or DROP ANY TABLE) on a table. To use the PURGE command, you need similar privileges. You must have the SELECT privilege and the FLASHBACK privilege on an object in order to query that object in the Recycle Bin.
Before Oracle Database 10g, if you suffered logical database corruption, you would undertake traditional point-in-time recovery techniques, restoring data file backup copies and then using archived
CHAPTER 16 DATABASE RECOVERY
redo logs to advance the database forward. This was often time-consuming and cumbersome. No matter how limited the extent of the corruption, you would need to restore entire data files and apply the archived redo logs.
Oracle can check data block integrity by computing checksums before writing the data blocks to disk. When the block is subsequently read again, the checksum for the data block is computed again, and if the two checksums differ, there is likely corruption in the data block. By setting the DB_BLOCK_CHECKSUM initialization parameter to FULL, you can make the database perform the check in the database buffer cache itself, thus eliminating the possibility of corruption at the physical disk level. The DB_BLOCK_CHECKSUM parameter is FALSE by default.
In Oracle Database 10g, the Flashback Database feature restores data files but without requiring backup data files and using just a fraction of the archived redo log information. A Flashback Database operation simply reverts all data files of the database to a specified previous point in time. With Flashback Database, the time it takes to recover is directly proportional to the number of changes that you need to undo. Thus, it is the size of the error and not the size of the database that determines the time it takes to recover. This means that you can recover from logical errors in a fraction of the time perhaps as little as a hundredth of the time, depending on the size of the database that it would take using traditional methods.