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reclaom space from LOB table

Oracle Tips by Burleson Consulting
November 18, 2008

Question:  I have a LOB segment and the table is not releasing the space allocated to the LOB segment (a CLOB column).  How do I reclaim the space reserved for a LOB after the LOB has been deleted?

Answer:  You can reclaim the space reserved for a LOB segment in several ways, the nost comon beuibng a reorganization of the table (using dbms_redefinition or data pump).

If your in-line LOB columns are frequently deleted, consider using BFILE. With out-of-line storage, UNDO is written only for the LOB locator and LOBINDEX changes.

You can also add the RETENTION table parameter which will re-use LOB space after the UNDO_RETENTION seconds.

You can also manage LOB space with the PCTVERSION table parameter which specifies the percentage of all used LOB data space that can be occupied by old versions of LOB data pages. As soon as old versions of LOB data pages start to occupy more than the PCTVERSION amount of used LOB space, Oracle tries to reclaim the old versions and reuse them.

Here are some way to re-claim LOB storage in a table:

ALTER TABLE xxx SHRINK SPACE CASCADE;

You can also reorg the individual LOB column:

alter table aqtab drop column lob_col;
alter table aqtab add (new_lob_col clob);
update aqtab set new_lob_col = lob_col;
alter table aqtab drop column lob_col;
alter table aqtab rename new_lob_col to lob_col;



 

 

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