CHAPTER 11
Large Objects
Need for Large Objects
• Data type to store objects that contain large amount of
text, log, image, video, or audio data.
• Most companies have a need to store this type of data.
• Oracle provides the following data types:
• LONG and LONG RAW
• Character large object (CLOB)
• National character large object (NCLOB)
• Binary large object (BLOB)
• Binary file (BFILE)
Understanding the Lob Architecture
Understanding BFILEs
BasicFiles versus SecureFiles
• SecureFile is a new LOB architecture introduced in Oracle
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Database 11g.
In Oracle Database 12c, default is to use SecureFiles
The SecureFile architecture has many new
enhancements that improve the manageability and
performance of LOBs.
If you’re not using Oracle Database 12c or 11g, then your
only option is to use the BasicFile architecture.
This is the default type of LOB created, and it’s been
available since Oracle version 8.
SecureFile LOB Enhancements
• Encryption (requires Oracle Advanced Security Option)
• Compression (requires Oracle Advanced Compression
Option)
• Deduplication (requires Oracle Advanced Compression
Option)
Prerequisites for SecureFiles
• A SecureFile LOB must be stored in a tablespace using
the automated segment space management feature
(ASSM).
• The DB_SECUREFILE initialization setting must be either
PERMITTED or ALWAYS.
Creating a BasicFile LOB Column
• Basic example:
create table patchmain(
patch_id number
,patch_desc clob);
• In prior code, table and LOB stored in the same
tablespace.
Default Behavior of LOBs
• LOBs by default are created as BasicFiles.
• Oracle creates a LOB segment and a LOB index for each LOB
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column.
The LOB segment has a name of this format: SYS_LOB<string>.
The LOB index has a name of this format: SYS_IL<string>.
The <string> is the same for each LOB segment and its associated
index.
The LOB segment and index are created in the same tablespace as
the table unless you specify a different tablespace.
By default, nearly 4000 bytes of a LOB are stored in the table row
(inline).
With Oracle Database 11g release 2 and higher, a LOB segment and
a LOB index aren’t created until a record is inserted into the table (the
so-called deferred segment creation feature). This means
DBA/ALL/USER_SEGMENTS and DBA/ALL/USER_EXTENTS have
no information in them until a row is inserted into the table.
Creating a LOB in a Different Tablespace
from the Table
• Allows you to maintain and manage LOB segment
separately from table segment and extents.
create table patchmain
(patch_id number
,patch_desc clob
,patch
blob
) tablespace users
lob (patch_desc) store as (tablespace clob_data)
,lob (patch)
store as (tablespace blob_data);
Creating a SecureFile LOB
• Use the SECUREFILE clause
• Must be in an ASSM managed tablespace
create table patchmain(
patch_id number
,patch_desc clob)
lob(patch_desc) store as securefile (tablespace lob_data);
Partitioning a LOB
• Allows you to spread out data across several tablespaces.
CREATE TABLE patchmain(
patch_id NUMBER
,region VARCHAR2(16)
,patch_desc CLOB)
LOB(patch_desc) STORE AS (TABLESPACE patch1)
PARTITION BY LIST (REGION) (
PARTITION p1 VALUES ('EAST')
LOB(patch_desc) STORE AS SECUREFILE
(TABLESPACE patch1 COMPRESS HIGH)
TABLESPACE inv_data1
,
PARTITION p2 VALUES ('WEST')
LOB(patch_desc) STORE AS SECUREFILE
(TABLESPACE patch2 DEDUPLICATE NOCOMPRESS)
TABLESPACE inv_data2
,
PARTITION p3 VALUES (DEFAULT)
LOB(patch_desc) STORE AS SECUREFILE
(TABLESPACE patch3 COMPRESS LOW)
TABLESPACE inv_data3
);
Moving a LOB
• Allows you to move a LOB to a separate tablespace from
the table.
alter table patchmain
move lob(patch_desc)
store as basicfile (tablespace inv_clob);
Adding a LOB Column
• Business requirements frequently change.
• You can add a column with a LOB datatype just like other
datatypes.
• Use ALTER TABLE ... ADD
SQL> alter table inv add(inv_image blob);
Removing a LOB Column
• Business requirements frequently change.
• Might want to rename the column before dropping it (to
better determine if an application is using the column):
SQL> alter table patchmain rename column patch_desc to
patch_desc_old;
SQL> alter table patchmain drop(patch_desc_old);
Caching LOBs
• By default, LOBs are not placed in the SGA.
• Use CACHE to place LOB in buffer cache for
reads/writes.
• Use CACHE READS to place LOB in buffer cache for only
reads.
• NOCACHE is the default, the LOB is not placed in the
buffer cache.
create table patchmain(
patch_id number
,patch_desc clob)
lob(patch_desc) store as (tablespace lob_data cache);
Storing LOBs In Line
• By default, up to approximately 4000 characters of a LOB column are
stored in line with the table row.
• If the LOB is over 4000 characters, then Oracle automatically stores
the LOB outside of the row data.
• Main advantage of storing a LOB in row is that small LOBs (less than
4000 characters) require less I/O, because Oracle doesn’t have to
search out of row for the LOB data.
create table patchmain(
patch_id number
,patch_desc clob
,log_file blob)
lob(patch_desc, log_file)
store as (
tablespace lob_data
enable storage in row);
Storing LOBs Out of Line
• You can explicitly instruct Oracle to store the LOB outside
of the row with the DISABLE STORAGE IN ROW.
create table patchmain(
patch_id number
,patch_desc clob
,log_file blob)
lob(patch_desc, log_file)
store as (
tablespace lob_data
disable storage in row);
SecureFile Compression
• If you’re using SecureFile LOBs, then you can specify a
degree of compression.
• The benefit is that the LOBs consume much less space in
the database.
• The downside is that reading and writing the LOBs may
take longer.
ALTER TABLE patchmain
MODIFY LOB(patch_desc)
(COMPRESS HIGH);
SecureFile Deduplicating
• If you have an application where identical LOBs are
associated with two or more rows, you should consider
using the SecureFile deduplication feature.
• When enabled, this instructs Oracle to check when a new
LOB is inserted into a table and see whether that LOB is
already stored in another row (for the same LOB column).
• If it’s already stored, then Oracle stores a pointer to the
existing identical LOB.
• This can dramatically reduce space for applications that
tend to store the same LOB value multiple times in
multiple rows.
SecureFile Deduplicating
CREATE TABLE patchmain(
patch_id NUMBER
,patch_desc CLOB)
LOB(patch_desc) STORE AS SECUREFILE
(DEDUPLICATE)
TABLESPACE inv_clob;
SecureFile Encryption
• You can transparently encrypt a SecureFile LOB column
(just like any other column).
• Before you use encryption features, you must set up an
encryption wallet.
CREATE TABLE patchmain(
patch_id number
,patch_desc clob)
LOB(patch_desc) STORE AS SECUREFILE (encrypt)
tablespace inv_clob;
Migrating BasicFiles to SecureFiles
• Create a new table, load the data from the old table, and
rename the tables.
• Move the table.
• Redefine the table online.
Viewing LOB Metadata
• Troubleshooting often begins by looking at LOB metadata.
select
table_name,column_name,index_name
,tablespace_name
from all_lobs
order by table_name;
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select
segment_name
,segment_type
,tablespace_name
from user_segments
where segment_name like 'SYS_LOB%'
or segment_name like 'SYS_IL%';
Loading a CLOB or BLOB
1. Create directory
2. Create table
3. Load LOB into table using DBMS_LOB package
Troubleshooting LOB space related
issues
select
a.table_name
,a.column_name
,a.segment_name
,a.index_name
,b.bytes/1024/1024 meg_bytes
from user_lobs a
,user_segments b
where a.segment_name = b.segment_name;
Determining Blocks used by LOB
• DBMS_SPACE.SPACE_USAGE for blocks being used by
a LOB.
• Overloaded procedure, one form for BasicFile the other
for SecureFile
Summary
• LOBs are a datatype you can use to store large objects
such as text files, images, video, and so on.
• Most companies have a need to store LOBs.
• Use the SecureFile feature from 11g forward.
• SecureFile is the new architecture and allows for
advanced LOB management.