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Streams log miner dictionary

Oracle Tips by Burleson 

Oracle Streams
Capture and Propagate Configuration

LogMiner Data Dictionary

The Capture process makes use of a data dictionary, which is different from the source database data dictionary. This special data dictionary is called the LogMiner Data Dictionary. The Capture process requires a LogMiner data dictionary because the information in the primary data dictionary may not match the changes being captured from the redo log. These changes may have occurred minutes, hours, or even days before they are captured by a Capture process. This is important in the sense that when the actual capture takes place, it needs to have the right dictionary information. Since the timing of the Capture process and timing of the actual database table change can be different, the maintenance of the log dictionary, which can provide time sensitive dictionary information, becomes significant.

The first time a Capture process is started at the database, Oracle uses the extracted data dictionary information in the redo log to create a LogMiner data dictionary.  This dictionary is separate from the primary data dictionary for the source database. Additional Capture processes may use this existing LogMiner data dictionary, or they may create new LogMiner data dictionaries.

Since the Capture process can have its own timing, when it does extract and create the LCRs, the object’s structure in the primary dictionary may not match. The Capture process cannot depend on the primary data dictionary, especially when it is stopped for some time. This is the reason that the LogMiner Data Dictionary is maintained.


LogMiner Data Dictionary

The procedure extracts data dictionary information to the redo log. The extracted data dictionary information in the redo log is consistent with the primary data dictionary at the time when the procedure is run. The build procedure is automatically run when the Capture process is created.

The amount of information extracted to a redo log, when the procedure is run, depends on the number of database objects in the database. The procedure generates a corresponding valid first SCN value that can be specified when the new Capture process is created. A First SCN generated by the procedure can be found by running the following query:


Propagation and Apply processes use a Streams data dictionary to keep track of the database objects from a particular source database. A Streams data dictionary is populated whenever one or more database objects are prepared for instantiation at a source database. When a database object is prepared for instantiation, it is recorded in the redo log. When a Capture process scans the redo log, it uses this information to populate the local Streams data dictionary for the source database. After an object has been prepared for instantiation, the local Streams data dictionary is updated whenever a DDL statement on the object is processed by a Capture Process.

The above text is an excerpt from:

Oracle Streams
High Speed Replication and Data Sharing

ISBN 0-9745993-5-2

by Madhu Tumma

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