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Lesson 7 Using LOB support with replication
Objective Describe the enhanced LOB Support in Oracle

Enhanced Oracle LOB Support

Oracle' s multimaster replication[1] originally supported the replication of tables that contained regular Oracle data types such as VARCHAR and NUMBER. This functionality has been extended in Oracle to support new data types that include large objects such as the LOB and BLOB data types. Multimaster replication now supports binary large objects (BLOBs) character large objects (CLOBs), and national character large objects (NCLOBs).

Oracle does not support replication of LOBs in distributed environments where some databases are running Oracle7.3 and below. All databases MUST be Oracle. This feature allows the replication of binary image data such as photographs and many other types of data, such as audio and video clips.
However, even with these enhancements, Oracle does not support the replication of columns that use the LONG and LONG RAW data types. Oracle simply omits columns containing these data types from replicated tables. Also, Oracle does not support LOB replication for BFILE columns.
Regardless, this new feature dramatically enhances Oracle's ability to support non-traditional data through snapshot and multimaster replication mechanisms. The next lesson shows you some new views for monitoring replication.

Large objects

The Oracle Database has been able to store large objects since Oracle8 added the capability to store multiple LOB columns in each table. Oracle Database 10g essentially removed the space limitation on large objects. Oracle Database 11g greatly improved the performance of query and insert operations when used with LOBs through the introduction of SecureFiles. SecureFiles serve as a place to securely store LOBs in the Oracle Database instead of in filesystems while delivering performance similar to that experienced when LOBs are stored in filesystems. Transparent Data Encryption, a security feature described below and later in the book, is supported for SecureFiles LOB data.

CLOB and NCLOB

The CLOB and NCLOB datatypes can store up to 4 GB of character data prior to Oracle Database 10g. Starting with Oracle Database 10g, the limit has been increased to 128 TBs, depending on the block size of the database. The NCLOB datatype stores the NLS data. Oracle Database 10g and later releases implicitly perform conversions between CLOBs and NCLOBs.

[1] Multimaster replication: Advanced replication features allowing updates on tables in remote sites as if every site were the master site.