Oracle Large Object (LOB) Support

Oracle LOB data can exist in several formats in Oracle.

For those types stored in the database (all types except BFILE), Oracle records the content of the LOB in the redo log. The Replication Agent reads the LOB data from the redo log and submits the data for replication.

Because BFILE type data is stored outside of the database, the BFILE contents are not recorded in the redo log. To replicate the content of a BFILE, the Replication Agent connects to the primary Oracle database and issues a query to select the data from the BFILE. Selecting the BFILE data separately from other data in the redo log can provide a temporary out-of-sync condition if the BFILE contents are changed multiple times.

• Replication of LOB Columns
  Oracle logs all LOB data (except for BFILE datatypes) in the Oracle redo log. This allows the Replication Agent to apply each individual LOB change. However, for BFILE data, the same technique is used and the same limitation exists—BFILE data is not logged but read from the database at the time the rest of the transaction is processed.
• Replicating CLOB and NCLOB Datatypes
  Oracle NCLOB (National Character Large Object) is a datatype that stores large character data using a multibyte national character set. Similarly, the CLOB datatype may also store character data using a multibyte national character set, when the Oracle database is defined with a double-byte or variable-width character set.