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Function documentation Logging Transactions Locate the document in the library structure

Use

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

Undo Log Management

Enables the rollback of transactions that have not yet been completed using undo log entries

Redo Log Management

Enables the repeat (redo) of transactions that have not yet been completed using undo log entries

History Management

Enables consistent views

Activities

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.

Note

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

This graphic is explained in the accompanying text

To configure the logging of transactions, use the log settings and database parameters.

Note

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

Transactions

 

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