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 Redo log content format tips

Oracle Database Tips by Donald BurlesonApril 30, 2015

Question:  I want to understand if an archived redo log contains the text of the DML changes go the database or whether the redo log has change vectors only.  How do I see the contents of a redo log?  to my understanding it contain the change vector of the changed block and the undo block vector information.

When we recover in instance recovery it may undo the uncommitted changes. Based on the SCN it recovers the committed blocks and rollbacks the data which is uncommitted using the undo block entry. This is all thru the information in the redo log file. Somewhere in the documentation it stated that from 10g that redo logs contain text of SQL as well as DML.

Answer:  The redo logs contains ONLY the changes to data (DDL and DML statements).  The 12c docs note these details on the contents of redo logs:

"Redo log files are filled with redo records. A redo record, also called a redo entry, is made up of a group of change vectors, each of which is a description of a change made to a single block in the database.

For example, if you change a salary value in an employee table, you generate a redo record containing change vectors that describe changes to the data segment block for the table, the undo segment data block, and the transaction table of the undo segments.

Redo entries record data that you can use to reconstruct all changes made to the database, including the undo segments. Therefore, the redo log also protects rollback data. When you recover the database using redo data, the database reads the change vectors in the redo records and applies the changes to the relevant blocks.

As we see, the redo logs do NOT contain any read-only SQL select statements.

They do not contain plain text either. You need the LogMiner utility to convert the DML and DDL into a readable format,  See my notes on using Oracle LogMiner.

Please read my concepts for Oracle redo logs
.

Regarding the contents of the redo logs, why guess about the redo log contents, take a look inside a redo log and see the vector data!

In UNIX/Linux, you can simply grab an archived redo log file, copy it to /tmp, and use the "strings" command to view the printable content:

strings mysid_12345.arc

You will see a printable character, here and there, but it is largely an unprintable vector format.  That is why you need a LogMiner dictionary to translate a redo log into readable form .

 

   
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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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