Question: How the
the LOB retention storage work in 11g?
Answer: (Brian Peasland) In the
olden days, we would use the PCTVERSION storage parameter
for their LOB segments to reserve a percentage of storage
space for read consistency of LOB segments.
Starting with Oracle 11g, you can now use the RETENTION
The RETENTION parameter will use the
UNDO_RETENTION parameter for determining how long to keep
LOB data for read-consistency purposes. But please be
advised that the RETENTION parametervdoes not use the Undo
tablespace, and the LOB segment’s tablespace is used for
When you change the
UNDO_RETENTION parameter, the LOB segment’s retention value
is not modified. If you query the RETENTION column of the
DBA_LOBS view, you will notice the old UNDO_RETENTION value
still remains after you have modified the UNDO_RETENTION
parameter. To change the LOB segment’s RETENTION value to
match the new UNDO_RETENTION value, do the following:
ALTER TABLE my_table MODIFY LOB (lob_column) (PCTVERSION
ALTER TABLE my_table MODIFY LOB (lob_column)
By momentarily changing
the LOB to use PCTVERSION and then back to RETENTION, the
new value of UNDO_RETENTION will be used. You need to do
this for all LOB segments that you intend to modify.
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