【MOS】EVENT: DROP_SEGMENTS - Forcing cleanup of TEMPORARY segments (文档 ID 47400.1)

 The DROP_SEGMENTS event
~~~~~~~~~~~~~~~~~~~~~~~
 Available from 8.0 onwards.
 

   DESCRIPTION Finds all the temporary segments in a tablespace which are not
     currently locked and drops them.
     For the purpose of this event a "temp" segment is defined as a 
     segment (seg$ entry) with TYPE#=3. Sort space in a TEMPORARY
     tablespace does not qualify under this definition as such
     space is managed independently of SEG$ entries.

   PARAMETERS
     level - tablespace number+1. If the value is 2147483647 then
             temp segments in ALL tablespaces are dropped, otherwise, only
             segments in a tablespace whose number is equal to the LEVEL
             specification are dropped.

   NOTES
     This routine does what SMON does in the background, i.e. drops
     temporary segments. It is provided as a manual intervention tool which
     the user may invoke if SMON misses the post and does not get to
     clean the temp segments for another 2 hours. We do not know whether
     missed post is a real possibility or more theoretical situation, so
     we provide this event as an insurance against SMON misbehaviour.

     Under normal operation there is no need to use this event.

     It may be a good idea to 
        alter tablespace <tablespace> coalesce;
     after dropping lots of extents to tidy things up.

 *SQL Session (if you can connect to the database):      
    alter session set events 'immediate trace name DROP_SEGMENTS level TS#+1';

     The TS# can be obtained from v$tablespace view:
     select ts# from v$tablespace where name = '<Tablespace name>';

     Or from SYS.TS$:

     select ts# from sys.ts$ where name = '<Tablespace name>' and online$ != 3;
     
     If ts# is 5, an example of dropping the temporary segments in that tablespace 
     would be:

    alter session set events 'immediate trace name DROP_SEGMENTS level 6'; 

Temporary Segment Cleanup

Oracle Database often requires temporary workspace for intermediate stages of SQL statement execution. Typical operations that may require a temporary segment include sorting, hashing, and merging bitmaps. While creating an index, Oracle Database also places index segments into temporary segments and then converts them into permanent segments when the index is complete.

Oracle Database does not create a temporary segment if an operation can be performed in memory. However, if memory use is not possible, then the database automatically allocates a temporary segment on disk.

Temporary segments will also be created for the following operations a well.

• CREATE TABLE AS SELECT
• ALTER INDEX REBUILD
• DROP TABLE
• CREATE SNAPSHOT
• CREATE PARTITION TABLE

When does SMON cleanup temporary segments?