Oracle actually introduced a "no change" optimisation for redo in 12.2
If an update doesn't change the values in the row then Oracle doesn't go
through the normal generation of undo and redo for that row. HOWEVER - and
this is where the "don't change the block" idea fails - it locks the row,
which does generate undo and redo. And if you're unlucky (and if you're
updating every row in the block in a small way you probably will be) the
"optimisation" will generate more undo and redo rather than less because
real updates can generate one undo and redo vector per block but the lock
row vectors are (up to 19.3) single row vectors.
I wrote about this a couple of years ago:
https://jonathanlewis.wordpress.com/2019/09/08/quiz-night-34/
The "lock row" is necessary for backwards compatibility (if nothing else) -
but I think it would be hard to argue that it shouldn't happen because if
the row were not locked then it would be possible to create cases that
produced wrong results.
Regards
Jonathan Lewis
On Thu, 17 Jun 2021 at 21:12, Mark W. Farnham <mwf@xxxxxxxx> wrote:
While I agreed with what JL wrote, IF we knew how fast processors are now,
then an update all rows query might compare the block images on the fly and
not write the blocks where all the rows in a block were updated to the same
value so that the actual block image resulting from the update didn’t need
to be written.
Off the top of my head I **think** that would work and be efficient, but
I am not engaged enough to ponder possible edge cases. But I **think**
you have all the information you need in hand at commit time to flag blocks
to **not** bother writing. If the buffer had to flush along the way
before the commit, they would already be in the redo stream, but with the
massive memory now available a private redo thread might be able to handle
it both correctly and efficiently.
I really like Oracle’s redo model though. Someone once put it to me
thusly: Everything Bill Bridge starts ends up in checkmate. (Meaning Bill
wins.) Making sure the bucket has no leaks was definitely priority number
one.
*From:* oracle-l-bounce@xxxxxxxxxxxxx [mailto:
oracle-l-bounce@xxxxxxxxxxxxx] *On Behalf Of *Jonathan Lewis
*Sent:* Thursday, June 17, 2021 10:16 AM
*To:* ORACLE-L (oracle-l@xxxxxxxxxxxxx)
*Subject:* Re: Original design approach to Oracle REDO Logs
Mike,
I don't think you can ask about hypothetical strategies for the redo log
before stating what you think the purpose of the database should be.
If you expect the database to be the reference of last resort for the
correctness and (immediate) consistency of the data then you might like a
different strategy from someone who thinks the database is some sort of
record of evolution of the data that allows the current version of the data
to be re-imaged by a client.
Regards
Jonathan Lewis
On Thu, 17 Jun 2021 at 13:55, Michael D O'Shea/Woodward Informatics Ltd <
woodwardinformatics@xxxxxxxxxxxxxxxx> wrote:
Chaps, request for opinions/discussion/feedback .....
Question: If Oracle were written today, would the same strategy behind the
"redo log" be adopted.
An example .... for a table of 10,000,000 rows, and a contrived piece of
DML
update someTable
set someColumn1 = 1.234
that "updated" all the rows
*but* where someColumn1 was only updated to 1.234 for 6 rows as the
remaining 10,000,000 - 6 rows were already 1.234
Should (assuming just DML and also just the basic data types, number,
varchar2, date, ... ) the redo log
* record the data "change" for all 10,000,000 "updates"
* record the real data change for just the 6 real updates
* record *solely* the SQL used to perform the update for some playback
purpose
* send a message to some message broker such as Solace, Tibco, ....
allowing subscribers to process the data change that way (might be
replication/backup, some sort of refresh or push notification to other
applications, email dispatch and so on)
* some other approach else
There is a considerable movement to event streaming technology such as
Kafka to (indirectly) drive data change events to downstream and dependent
systems (in Oracle assumably by polling the redo log file, or maybe some
LogMiner interface .. I don't know the detail) in databases that include
Oracle, MongoDB (referred to as Change Streams), and many more.
My "ask" focuses more on "just the database" interoperability with the
remainder of what is often a large tech stack, and not the original design
decision around redo logs for data recovery.
Mike
http://www.strychnine.co.uk
