Oracle Magazine - September/October 2007 - Marcelo Machado

DROP TABLE article_draft;
RENAME article_draft_redef
TO article_draft;
There’s really no need to take your
data offline, though. Instead, consider
using the online table redefinition
feature introduced in Oracle9i Database
Release 1. Listing 2 shows a PL/SQL
block that migrates the article_draft
table to the structure indicated by the
article_draft_redef table. Online table
definition is very easy to do, and it
makes the migration transparent to
users and applications, because the
table being migrated remains available
the entire time.
If your LOB table happens to be
partitioned, another approach to consider
is partition exchange. You can
specify LOB storage on a perpartition
basis, giving you the option
of migrating one partition at a time
from BasicFiles to SecureFiles. Use the
URL under “nextSTEPS” to go to Arup
Nanda’s article on partitioning features,
“Partition Decisions,” where you will
find an example showing how partition
exchange is done.
Be sure you have enough tablespace
and disk space set aside to support
whatever migration method you
choose. For the online redefinition
approach, you’ll need enough space
for two complete copies of your data
to coexist. Partition exchange requires
only enough disk space to hold two
copies of whichever partition you are
exchanging. Bear in mind that one
downside of partition exchange is that
the partition being swapped needs to
be briefly taken offline. In the end,
you’ll have to weigh the different
approaches and their trade-offs and
choose the approach that works best in
your own situation.
CONFIRMING MIGRATION
You can confirm the SecureFiles status
of a given LOB column by checking
that column’s segment subtype. For
example, execute the following to
confirm that article_content is now a
SecureFiles LOB:
codeLISTING 2: Migrating by online redefinition
DECLARE
error_counter PLS_INTEGER;
BEGIN
--Begin the redefinition process
DBMS_REDEFINITION.START_REDEF_TABLE (
‘gennick’, ’article_draft’, ’article_draft_redef’,
‘article_id, article_stage, article_content’);
--Finish the redefinition process
DBMS_REDEFINITION.FINISH_REDEF_TABLE (
‘gennick’, ’article_draft’, ’article_draft_redef’);
END;
codeLISTING 3: Checking space used by a LOB
DECLARE
seg_blocks NUMBER;
seg_bytes NUMBER;
used_blocks NUMBER;
used_bytes NUMBER;
expired_blocks NUMBER;
expired_bytes NUMBER;
unexpired_blocks NUMBER;
unexpired_bytes NUMBER;
BEGIN
DBMS_SPACE.SPACE_USAGE (
‘GENNICK’, ‘ARTICLE_CONTENT’, ‘LOB’
, seg_blocks, seg_bytes, used_blocks, used_bytes
, expired_blocks, expired_bytes, unexpired_blocks, unexpired_bytes);
DBMS_OUTPUT.PUT_LINE (‘Bytes used = ‘ || to_char(used_bytes));
END;
SELECT segment_subtype
FROM user_segments
WHERE segment_name=’ARTICLE_CONTENT’;
The result you want to see is
SECUREFILE, which indicates
SecureFiles storage. If you see ASSM,
you’re still set for BasicFiles storage.
ENABLING DEDUPLICATION
Having migrated to Oracle SecureFiles,
you can choose to enable deduplication,
an Oracle SecureFiles feature whereby
the database server stores only one
copy of a given LOB in a given column
within the same partition. Two or more
users can independently store the same
data in a LOB column—storing the
same article draft twice, for example—
and the database keeps track of that
duplication, storing only one copy of
the data. Deduplication is transparent.
Users each perceive that they have their
own copy of deduplicated data, even
though that is not really the case.
In a real-life migration, it would
likely make sense to deduplicate during,
not after, the migration process. For
example purposes, though, it’s nice to
be able to see that removal of duplicates
is actually occurring. Run the
code in Listing 3 to see how many
bytes are in use by the LOB segment.
(Be sure to change the first parameter
of the procedure call to a valid schema
name on your system.) On my system,
the result is
Bytes used = 450560
To deduplicate the data in the
article_content column in the
article_draft table, issue the following
ALTER TABLE statement:
ORACLE MAGAZINE SEPTEMBER/OCTOBER 2007 79