Oracle: When to use Log Miner

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Log Miner has both a GUI interface in OEM as well as the database package, DBMS_LOGMNR. When this utility is used by the DBA, its primary focus is to mine data from the online and archived redo logs. Internally Oracle uses the Log Miner technology for several other features, such as Flashback Transaction Backout, Streams, and Logical Standby Databases. This section is not on how to run Log Miner, but looks at the task of identifying the information to restore.

The Log Miner utility comes into play when you need to retrieve an older version of selected pieces of data without completely recovering the entire database. A complete recovery is usually a drastic measure that means downtime for all users and the possibility of lost transactions. Most often Log Miner is used for recovery purposes when the data consists of just a few tables or a single code change.

Make sure supplemental logging is turned on (see the Add Supplemental Logging section). In this case, you discover that one or more of the following conditions apply when trying to recover a small amount of data that was recently changed:

  • Flashback is not enabled
  • Flashback logs that are needed are no longer available
  • Data that is needed is not available in the online redo logs
  • Data that is needed has been overwritten in the undo segments

Go to the last place available: archived redo logs. This requires the database to be in archivelog mode and for all archive logs that are needed to still be available or recoverable.

Identifying the data needed to restore

One of the hardest parts of restoring data is determining what to restore, the basic question being when did the bad data become part of the collective? Think the Borg from Star Trek! When you need to execute Log Miner to retrieve data from a production database, you will need to act fast. The older the transactions the longer it will take to recover and traverse with Log Miner. The newest (committed) transactions are processed first, proceeding backwards. The first question to ask is when do you think the bad event happened? Searching for data can be done in several different ways:

  • SCN, timestamp, or log sequence number>
  • Pseudo column ORA_ROWSCN

SCN, timestamp, or log sequence number

If you are lucky, the application also writes a timestamp of when the data was last changed. If that is the case, then you determine the archive log to mine by using the following queries. It is important to set the session NLS_DATE_FORMAT so that the time element is displayed along with the date, otherwise you will just get the default date format of DD-MMM-RR. The data format comes from the database startup parameters— the NLS_TERRITORY setting. Find the time when a log was archived and match that to the archive log needed.

Pseudo column ORA_ROWSCN

While this method seems very elegant, it does not work perfectly, meaning it won’t always return the correct answer. As it may not work every time or accurately, it is generally not recommended for Flashback Transaction Queries. It is definitely worth trying to narrow the window that you will have to search. It uses the SCN information that was stored for the associated transaction in the Interested Transaction List.

You know that delayed block cleanout is involved. The pseudo column ORA_ROWSCN contains information for the approximate time this table was updated for each row. In the following example the table has three rows, with the last row being the one that was most recently updated. It gives me the time window to search the archive logs with Log Miner.

Log Miner is the basic technology behind several of the database Maximum Availability Architecture capabilities—Logical Standby, Streams, and the following Flashback Transaction Backout exercise.

Flashback Transaction Query and Backout

Flashback technology was first introduced in Oracle9i Database. This feature allows you to view data at different points in time and with more recent timestamps (versions), and thus provides the capability to recover previous versions of data. In this article, we are dealing with Flashback Transaction Query (FTQ) and Flashback Transaction Backout (FTB), because they both deal with transaction IDs and integrate with the Log Miner utility. See the MOS document: “What Do All 10g Flashback Features Rely on and what are their Limitations?” (Doc ID 435998.1).

Flashback Transaction Query uses the transaction ID (Xid) that is stored with each row version in a Flashback Versions Query to display every transaction that changed the row. Currently, the only Flashback technology that can be used when the object(s) in question have been changed by DDL is Flashback Data Archive. There are other restrictions to using FTB with certain data types (VARRAYs, BFILES), which match the data type restrictions for Log Miner. This basically means if data types aren’t supported, then you can’t use Log Miner to find the undo and redo log entries.

When would you use FTQ or FTB instead of the previously described methods? The answer is when the data involves several tables with multiple constraints or extensive amounts of information. Similar to Log Miner, the database can be up and running while people are working online in other schemas of the database to accomplish this restore task.

An example of using FTB or FTQ would be to reverse a payroll batch job that was run with the wrong parameters. Most often a batch job is a compiled code (like C or Cobol) run against the database, with parameters built in by the application vendor. A wrong parameter could be the wrong payroll period, wrong set of employees, wrong tax calculations, or payroll deductions.

Enabling flashback logs

First off all flashback needs to be enabled in the database. Oracle Flashback is the database technology intended for a point-in-time recovery (PITR) by saving transactions in flashback logs. A flashback log is a temporary Oracle file and is required to be stored in the FRA, as it cannot be backed up to any other media. Extensive information on all of the ramifications of enabling flashback is found in the documentation labeled: Oracle Database Backup and Recovery User’s Guide.

See the following section for an example of how to enable flashback:

SYS@NEWDB>ALTER SYSTEM SET DB_RECOVERY_FILE_DEST='/backup/flash_
recovery_area/NEWDB' SCOPE=BOTH;
SYS@NEWDB>ALTER SYSTEM SET DB_RECOVERY_FILE_DEST_SIZE=100M SCOPE=BOTH;
--this is sized for a small test database
SYS@NEWDB> SHUTDOWN IMMEDIATE;
SYS@NEWDB> STARTUP MOUNT EXCLUSIVE;
SYS@NEWDB> ALTER DATABASE FLASHBACK ON;
SYS@NEWDB> ALTER DATABASE OPEN;
SYS@NEWDB> SHOW PARAMETER RECOVERY;

The following query would then verify that FLASHBACK had been turned on:

SYS@NEWDB>SELECT FLASHBACK_ON FROM V$DATABASE;

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