How To Reclaim Wasted Space on The Segment (Table, Index and LOB) and Tablespace Levels (文档 ID 16827

How To Reclaim Wasted Space on The Segment (Table, Index and LOB) and Tablespace Levels (文档 ID 1682748.1)

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In this Document Purpose   Details   How to know if the segment is fragmented   Table   Index   LOB segment (both Securefiles and Basic files)    How to reclaim free space (below HWM)   Table   Index   LOB Segment   How to reclaim the space back to the OS / ASM diskgroup by decreasing the datafile size   System tablespace   References APPLIES TO: Oracle Database - Enterprise Edition - Version 10.2.0.1 and later Information in this document applies to any platform. PURPOSE   This note aims to provide guidance on how to reclaim wasted space (fragmented space) introduced by deleting rows, moving or dropping tables, etc .. DETAILS How to know if the segment is fragmented Table Use Segment advisor (best and most accurate results) . Refer    10g and above SEGMENT ADVISOR (Doc ID 242736.1)    Automatic Segment Advisor in Oracle 10g Release 2 (10.2) (Doc ID 314112.1) Use the following query (Statistics must be fresh for the tables to be examined) exec dbms_stats.gather_table_stats('<OWNER>','<TABLE NAME>'); select owner,table_name,round((blocks*8),2)||' kb' "TABLE SIZE",round((num_rows*avg_row_len/1024),2)||' kb' "ACTUAL DATA" from dba_tables where table_name='<YOUR TABLES'S NAME>'; Index An index could grow to be even bigger than the table's size at times, if your table has only one column then it's expected than the index's size could slightly exceed the table's size. The other reason for index growth (bigger than table's size or not) would be fragmentation. Rebuilding an index or recreating it usually save space on the cost of index performance. Before you decide to rebuild / recreate and index please check the following document first: Index Rebuild, the Need vs the Implications (Doc ID 989093.1) LOB segment (both Securefiles and Basic files)  You can use the function DBMS_SPACE.SPACE_USAGE . This is applicable only for LOBs on ASSM tablespaces. You can calculate the actual size of data and see if it's hugely lower than the size of the extent reported by DBA_EXTENTS. This is applicable on both ASSM and MSSM tablespaces. The details for both methods are explained in the following document: How to Determine what storage is used in a LOBSEGMENT and should it be shrunk / reorganized? (Doc ID 1453350.1) How to reclaim free space (below HWM) Table The first method is by recreating the table, there are many alternatives to accomplish this task: Create Table As Select (CTAS) Export / Import Alter table ... move Online Redefinition (only method that fully allows DMLs to run against the table while the operation is running) This is discussed in details in the following note:  How to Reorganize a Table (Doc ID 151588.1) The second method is by using the shrink command             Refer: Why is no space released after an ALTER TABLE ... SHRINK? (Doc ID 820043.1) Index Use alter index .. rebuild command Use alter index .. coalesce command Drop and recreate the index LOB Segment There are multiple options here: Shrink command (not applicable for Securefile LOBs) DBMS_REDEFINITION (Fully online operation) Export / Import This is explained in more details in How to Shrink (make less sparse) a LOB (BASICFILE or SECUREFILE)? (Doc ID 1451124.1) How to reclaim the space back to the OS / ASM diskgroup by decreasing the datafile size If you want to decrease the datafile's size by an amount bigger than the bytes at the end of the datafile you will have to do the following procedure: 1- Run SHRINK_DATAFILE.SQL from How to Resize a Datafile (Doc ID 1029252.6) 2- You will provide two pieces of information to the script, the file ID and the desired new size 3- Move the segments that appear in the script's result set to a new tablespace 4- Run the script again and make it returns no results 5- Use the "ALTER DATABASE DATAFILE '<full path and name of the file>' RESIZE [K|M|G];" command to resize the datafile If you tried to resize the datafile beyond the the bytes at the end of the datafile you will hit an ORA-3297 error For more details please refer to How to Resize a Datafile (Doc ID 1029252.6) System tablespace Please note that the procedure described in the previous section doesn't apply to system tablespace, this tablespace contains many bootstrap objects that should not be moved, if system tablespace is fragmented the only possible way is to recreate the database. You can recreate the database and copy the data using TTS (Transportable Tablespace) or Export/Import method.