Hi reader,

with oracle 10.2 you have something like a segment fragment which could release space not needed anymore. First you enable row movement to enable reorganization. Then you start to shrink. With the dba_segments view you can query the segment properties of your table or index.

SELECT Us.Segment_Name,
       Us.Segment_Type,
       Us.Bytes,
       Us.Blocks
  FROM User_Segments Us
 WHERE Us.Segment_Name = ‘EVENTLOG’

;

SQL>                                      
SQL> SELECT Us.Segment_Name,
  2         Us.Segment_Type,
  3         Us.Bytes,
  4         Us.Blocks
  5    FROM User_Segments Us
  6   WHERE Us.Segment_Name = ‘EVENTLOG’
  7  ;

SEGMENT_NAME                                SEGMENT_TYPE            BYTES     BLOCKS
——————————————- —————— ———- ———-
EVENTLOG                                    TABLE                84934656      10368

SQL>

 

Like a diskdefragmenter must be able to move disk blocks of a file the table defragmentation only works effective if rows are allowed to be moved from one oracle block to another. This means the rowid of a table row changes.

ALTER TABLE EVENTLOG ENABLE ROW MOVEMENT

;
ALTER TABLE EVENTLOG SHRINK SPACE

;

SQL> ALTER TABLE EVENTLOG ENABLE ROW MOVEMENT;
Table altered
SQL> ALTER TABLE EVENTLOG SHRINK SPACE;
Table altered
SQL>

In this example the number blocks could be reduced from 10368 to 128! Also this means that the high water mark which is the end position of the Full Table Scan is reseted too causing reduced resource usage for Full Table Scans.

SQL>                                                                                
SQL> SELECT Us.Segment_Name,                                                        
  2         Us.Segment_Type,                                                        
  3         Us.Bytes,                                                               
  4         Us.Blocks                                                               
  5    FROM User_Segments Us                                                        
  6   WHERE Us.Segment_Name = ‘EVENTLOG’
  7  ;                                           
                                                                                    
SEGMENT_NAME                                SEGMENT_TYPE            BYTES     BLOCKS
——————————————- —————— ———- ———-
EVENTLOG                                    TABLE                 1048576        128
                                                                                    
SQL>                                                                                

How do you detect candidates for shrinking? simply look into Segment Advisor Recommendations of the database Homepage of the DbConsole or do this by SQL using the Dba_Advisor_Findings view :

SELECT Daf.Task_Id,
       Daf.Finding_Id,
       Daf.Task_Name,
       Daf.TYPE,
       Daf.Object_Id,
       Daf.Message
  FROM Dba_Advisor_Findings Daf
 WHERE Daf.Task_Name LIKE ‘SYS_AUTO_SPCADV%’
   AND Message LIKE ‘%EVENTLOG%’

;

SQL>
SQL> SELECT Daf.Task_Id,
  2         Daf.Finding_Id,
  3         Daf.Task_Name,
  4         Daf.TYPE,
  5         Daf.Object_Id,
  6         Daf.Message
  7    FROM Dba_Advisor_Findings Daf
  8   WHERE Daf.Task_Name LIKE ‘SYS_AUTO_SPCADV%’
  9     AND Message LIKE ‘%EVENTLOG%’
 10  ;

   TASK_ID FINDING_ID TASK_NAME                      TYPE         OBJECT_ID MESSAGE
———- ———- —————————— ———– ———- ——————————————————————————–
     14837         10 SYS_AUTO_SPCADV_250201882006   INFORMATION         28 Zeilenverschiebung der Tabelle EVENTLOG aktivieren und Verkleinerung vorneh
     14870         11 SYS_AUTO_SPCADV_25041982006    INFORMATION         28 Zeilenverschiebung der Tabelle EVENTLOG aktivieren und Verkleinerung vorneh
     15127          7 SYS_AUTO_SPCADV_410202182006   INFORMATION         23 Zeilenverschiebung der Tabelle EVENTLOG aktivieren und Verkleinerung vorneh
     15228          1 SYS_AUTO_SPCADV_921202282006   INFORMATION          1 Zeilenverschiebung der Tabelle EVENTLOG aktivieren und Verkleinerung vorneh

SQL>

HTH Karl

Links

• Segment Shrink