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Chapter 11

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

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Chapter 11

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  1. Chapter 11 Large Objects

  2. 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)

  3. Understanding the Lob Architecture

  4. Understanding BFILEs

  5. BasicFilesversus SecureFiles • SecureFile is a new LOB architecture introduced in Oracle Database 11g. • The SecureFile architecture has many new enhancements that improve the manageability and performance of LOBs. • If you’re using Oracle Database 11g or higher, then you should create your LOB columns with the SECUREFILE clause. • If you’re not using Oracle Database 11g, then your only option is to use the BasicFilearchitecture. • This is the default type of LOB created, and it’s been available since Oracle version 8.

  6. SecureFileLOB Enhancements • Encryption (requires Oracle Advanced Security Option) • Compression (requires Oracle Advanced Compression Option) • Deduplication (requires Oracle Advanced Compression Option)

  7. 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.

  8. Creating a BasicFile LOB Column • Basic example: create table patchmain( patch_id number ,patch_descclob); • In prior code, table and LOB stored in the same tablespace.

  9. Default Behavior of LOBs • LOBs by default are created as BasicFiles. • Oracle creates a LOB segment and a LOB index for each LOB 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.

  10. 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_descclob ,patch blob ) tablespace users lob (patch_desc) store as (tablespace clob_data) ,lob (patch) store as (tablespace blob_data);

  11. Creating a SecureFile LOB • Use the SECUREFILE clause • Must be in an ASSM managed tablespace create table patchmain( patch_id number ,patch_descclob) lob(patch_desc) store as securefile (tablespace lob_data);

  12. Partitioning a LOB • Allows you to spread out data across several tablespaces. CREATE TABLE patchmain( patch_id NUMBER ,region VARCHAR2(16) ,patch_descCLOB) 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 DEDUPLICATENOCOMPRESS) TABLESPACE inv_data2 , PARTITION p3 VALUES (DEFAULT) LOB(patch_desc) STORE AS SECUREFILE (TABLESPACE patch3 COMPRESS LOW) TABLESPACE inv_data3 );

  13. 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);

  14. 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);

  15. 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);

  16. 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_descclob) lob(patch_desc) store as (tablespace lob_data cache);

  17. Storing LOBsIn 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_descclob ,log_file blob) lob(patch_desc, log_file) store as ( tablespace lob_data enable storage in row);

  18. Storing LOBsOut 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_descclob ,log_file blob) lob(patch_desc, log_file) store as ( tablespace lob_data disable storage in row);

  19. SecureFile Compression • If you’re using SecureFileLOBs, 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);

  20. SecureFileDeduplicating • If you have an application where identical LOBs are associated with two or more rows, you should consider using the SecureFilededuplicationfeature. • 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.

  21. SecureFileDeduplicating CREATE TABLE patchmain( patch_id NUMBER ,patch_descCLOB) LOB(patch_desc) STORE AS SECUREFILE (DEDUPLICATE) TABLESPACE inv_clob;

  22. 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_descclob) LOB(patch_desc) STORE AS SECUREFILE (encrypt) tablespace inv_clob;

  23. 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.

  24. 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; • select • segment_name • ,segment_type • ,tablespace_name • from user_segments • where segment_name like 'SYS_LOB%' • or segment_name like 'SYS_IL%';

  25. Loading a CLOB or BLOB • Create directory • Create table • Load LOB into table using DBMS_LOB package

  26. 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;

  27. 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

  28. 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.

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