How To Efficiently Drop A Table With Many Extents

删除大表时，为了控制cpu消耗，我们可以采用以下办法，分步进行删除，以下信息摘自metalink ID 68836.1

Permanent object cleanup
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   If a permanent object (table) is made up of many extents, and the object is
   to be dropped, the user process dropping the object will consume large
   amounts of CPU - this is an inescapable fact. However, with some forethought
   it is possible to mitigate the effects of CPU usage (and hence the knock-on
   effect on other users of system resources) thus:

   1. Identify, but do NOT drop the table
   2. Truncate the table, specifying the REUSE STORAGE clause. This will be
      quick as extents are not deallocated; the highwater mark is simply
      adjusted to the segment header block.
   3. Deallocate unused extents from the table, SPECIFYING THE KEEP CLAUSE.
      This is the crux - you can control how many extents are to be deallocated
      by specifying how much (in terms of Kb or Mb) of the table is NOT
      to be deallocated.

   Example:
   o. Table BIGTAB is 2Gb in size and consists of 262144 8Kb extents
   o. There is little CPU power available, and (from past experience) it is
      known that dropping an object of this number of extents can take days
   o. The system is quiet at night times (no other users or batch jobs)
  
   In the above example the table could be dropped in 'phases' over the period
   of a few nights as follows:
   1. Truncate the table, specifying the REUSE STORAGE clause:
      SQL> TRUNCATE TABLE BIGTAB REUSE STORAGE;
   2. If it takes 3 days (72 hours) to drop the table, spread this out over
      6 nights i.e. drop 1/3 Gb per night. This can be achieved in 6 (nightly)
      steps as follows:
      Night 1:
        SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1707M; (2Gb*5/6)
      Night 2:
        SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1365M; (2Gb*4/6)
      Night 3:
        SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1024M; (2Gb*3/6)
      Night 4:
        SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 683M; (2Gb*2/6)
      Night 5:
        SQL> ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 341M; (2Gb*1/6)
      Night 6:
        SQL> DROP TABLE BIGTAB;

   The same method can be applied if LOB segments or indexes are involved.

        SQL> ALTER TABLE <table_name> MODIFY LOB (<lob_column>)
             DEALLOCATE UNUSED KEEP <value>M;

        SQL> ALTER INDEX <index_name> DEALLOCATE UNUSED KEEP <value>M;