The database system logs all transactions that change data in log entries. The database system uses these log entries to roll back individual SQL statements, if necessary, or to repeat them and thereby maintain transaction consistency.
Types of Logging
Name |
Purpose |
Enables the rollback of transactions that have not yet been completed using undo log entries |
|
Enables the repeat (redo) of transactions that have not yet been completed using undo log entries |
|
Enables consistent views |
At the beginning of a transaction that will change data, the database system writes undo log entries (before images) for all database objects that are to be changed by the transaction to an undo log file in the data area.
When such a transaction has been closed with a COMMIT, the database system writes redo log entries (after images) to the log queue in the main memory. The database system then writes the redo log entries from the log queue to the log area when a page in the log queue is full or a transaction was closed with a COMMIT.
The time intervals between the individual write operations from the log queue to the log area are much smaller than the intervals between savepoints.
Logging
To configure the logging of transactions, use the log settings and database parameters.
The database system can log not only transactions, but also other actions (see Log Files and Traces)
See also:
Using Log Entries for a Restart or Restore