Re: When an ITL entry can be reused??

When the transaction has committed, the ITL is marked with the SCN (for short transactions, though long transactions may skip marking the ITL and leave the cleanout to the next process that reads the block).

If a subsequent transaction uses the ITL, and a third process needs to see the original state of the block the steps go as follows (all actions taken by the 3rd process).

Copy block

Use ITL entry of second transaction to find rollback for second transaction and rollback the block. Part of the rollback information describes the state of the ITL at the moment the second transaction started.

i.e. the block not contains the ITL as left by the 1st transaction, which means it is now pointing to the rollback for the first transaction, so this transaction can now be rolled back too.

There are numerous boundary variations, but this is basically it - the cunning bit is that the rollback contains information about the previous state of the ITL.

--
Jonathan Lewis
Yet another Oracle-related web site:  http://www.jlcomp.demon.co.uk

Practical Oracle 8i:  Building Efficient Databases

Publishers:  Addison-Wesley
Book bound date: 8th Dec 2000
See a first review at:
http://www.ixora.com.au/resources/index.htm#practical_8i
More reviews at: http://www.jlcomp.demon.co.uk/book_rev.html



jrodon_at_isasa.es wrote in message <8vlbsh$n1i$1_at_nnrp1.deja.com>...


>Hi Oracle experts.

>

>I'm trying to understand how does it work Oracle to make read

>consistents.

>

>I know the existence of the locked flag in the row, the ITL table in the

>data block header and the table transaction in the rollback segment

>header. I understand too how to follow this sequence to find if the row

>is locked or not.

>

>But I don't understand when an ITL entry can be reused or a new entry

>must be created. If the transaction has commited this ITL entry can be

>reused??. And what about "consistent reads" for other transactions??

>

>Anybody can explain me this.

>

>Thanks in advance.

>

>

>Sent via Deja.com http://www.deja.com/

>Before you buy.