How the V$LOGMNR_CONTENTS View Is Populated

How the V$LOGMNR_CONTENTS View Is Populated
The V$LOGMNR_CONTENTS fixed view is unlike other views in that it is not a selective presentation of data stored in a table. Instead, it is a relational presentation of the data that you request from the redo log files. LogMiner populates the view only in response to a query against it. You must successfully start LogMiner before you can query V$LOGMNR_CONTENTS.
When a SQL select operation is executed against the V$LOGMNR_CONTENTS view, the redo log files are read sequentially. Translated information from the redo log files is returned as rows in the V$LOGMNR_CONTENTS view. This continues until either the filter criteria specified at startup are met or the end of the redo log file is reached.
In some cases, certain columns in V$LOGMNR_CONTENTS may not be populated. For example:
• The TABLE_SPACE column is not populated for rows where the value of the OPERATION column is DDL. This is because a DDL may operate on more than one tablespace. For example, a table can be created with multiple partitions spanning multiple table spaces; hence it would not be accurate to populate the column.
• LogMiner does not generate SQL redo or SQL undo for temporary tables. The SQL_REDO column will contain the string "/* No SQL_REDO for temporary tables */" and the SQL_UNDO column will contain the string "/* No SQL_UNDO for temporary tables */".
LogMiner returns all the rows in SCN order unless you have used the COMMITTED_DATA_ONLY option to specify that only committed transactions should be retrieved. SCN order is the order normally applied in media recovery.
See Also:
Showing Only Committed Transactions for more information about the COMMITTED_DATA_ONLY option to DBMS_LOGMNR.START_LOGMNR
Note:
Because LogMiner populates the V$LOGMNR_CONTENTS view only in response to a query and does not store the requested data in the database, the following is true:
• Every time you query V$LOGMNR_CONTENTS, LogMiner analyzes the redo log files for the data you request.
• The amount of memory consumed by the query is not dependent on the number of rows that must be returned to satisfy a query.
• The time it takes to return the requested data is dependent on the amount and type of redo log data that must be mined to find that data.
Querying V$LOGMNR_CONTENTS Based on Column Values
LogMiner lets you make queries based on column values. For instance, you can perform a query to show all updates to the hr.employees table that increase salary more than a certain amount. Data such as this can be used to analyze system behavior and to perform auditing tasks.
LogMiner data extraction from redo log files is performed using two mine functions: DBMS_LOGMNR.MINE_VALUE and DBMS_LOGMNR.COLUMN_PRESENT. Support for these mine functions is provided by the REDO_VALUE and UNDO_VALUE columns in the V$LOGMNR_CONTENTS view.