I was reading about Oracle's locking model, maybe someone here knows a bit about both and can clarify.

According to the article, Oracle doesn't block reads while an application updates rows! (or vice versa)

While I'm sure this is great for concurrency, it seems to me that it gives inaccurate results...

ie:
Oracle:
Application 1 - updates row (no commit)
Application 2 - reads row (gets old data!)

DB2:
Application 1 - updates row (no commit)
Application 2 - reads row, blocks
Application 1 - commit
Application 2 - gets new data

(Unless using UR)

So it seems like a fundamental difference as to when a change takes place... in DB2 the row is deemed to have changed when the statement is issued, in Oracle it's deemed to have changed when the statement is committed.

Is my understanding accurate?

I think your understanding is correct. It is recommended when developing with Oracle to re-read the record that you're about to update using "FOR UPDATE" clause - basically, use optimistic locking strategy.

Yes!

Your understanding is all correct. In Oracle, an update will create a new version of the data and a commit will allow others to see it. Old data can now discarded. However, for recovery needs Oracle needs also logging.
- If a rollback is needeed, this new data is just discarded.

In DB2 when you do an update, DB2 log saves old version and then data will be overwritten. However, others may not see this new data, until a commit is issued.
- If a rollback is needeed, DB2 fetch old version fron the log and rewrite old data back.


Cheers, Bill

Very interesting, thanks.

Stinger and DB2_EVALUNCOMMITTED registry variable. With this variable enabled, predicate evaluation can occur on uncommitted data. This means that a row that contains an uncommitted update may not satisfy the query, whereas if the predicate evaluation waited until the updated transaction completed, the row may satisfy the query.

How we can check the table locking in the DB2 Z OS? How we can avoid it?

If you are concerned about accuracy in an Oracle environment, you should know that Oracle has a SERIALIZABLE isolation level that is, somehow, similar to DB2 RR isolation level. Although the internal mechanisms for assuring isolation are quite different, yes.

The posts about Oracle (and DB2) prior to the most recent post about Db2 for z/OS before yours, are almost 5 years old.

Sorry I usually read the new post - so I didn't pay attention to the first posts' dates.