How does DB Operation takes place

Question

Hi Gurus,
I have a question in the DB Operation of Select.
Scenario:
    I have a SELECT Query on a table which runs for around 1 hour.
    After the select process is started at the 30th Minute there is an UPDATE Statement being fired on that table and committed.
    Now when the SELECT Query completes execution after 1 hour and returns the result, will it display the OLD Value of the record or the NEW Updated Value of the record.
My assumption is that when the SELECT query is fired it will fetch the data from DATA SEGMENTS and store it in the SGA Buffer Memory and then return the result.
When the Update statement fires it will update the data in the DATA SEGMENT. (more over i believe that the physical location of that record in data block would get changed during a UPDATE)
So when the SELECT statement completes execution it will take the data stored in SGA buffer and return the result.
Please confirm what is the correct answer. How does Oracle Architecture handles this.

Answer 1

Hi,
Ranganathan -Oracle wrote:
Yes Understood that.
So if that select statement which i use is for showing some data in Front End and if user see the OLD value and if he uses that value for manipulation, then the changes of that update statement will be lost and is mostly a Data Loss in the system.
Do we have any option to avoid this.
You can have this problem if you don't write good application.
The way to avoid this problem it's also mentioned in the page I have posted:
Oracle Database maintains data consistency by using a multiversion consistency model and various types of locks and transactions. In this way, the database can present a view of data to multiple concurrent users, with each view consistent to a point in time. Because different versions of data blocks can exist simultaneously, transactions can read the version of data committed at the point in time required by a query and return results that are consistent to a single point in time.
Let's say that you in your application you have a product list and you have an edit button beside each product. If you want to prevent other users to make a change until your transaction is completed you can lock the row when you enter into edit mode (i.e. SELECT FOR UPDATE) and release the lock (with commit or rollback) when the transaction is completed or cancelled.
This will ensure that the row cannot be updated by other transactions.
Regards.
Alberto