Oracle's locking strategy question

Question

I have a question related to Oracle's locking mechanism which I understand is related to taking "snapshots in time" to avoid writers blocking readers (which happens in SQL Server).
Suppose I had a table T1 with 1 billion records and column C1 = 4 in every row. Suppose the following events occur:
12:00 update T1 set C1 = 5
12:01 select C1 from T1 where ... (and this statement returns only one row)
1:00 the update statement executed at 12:00 finally finishes
Would oracle return the value 4 regardless or which row was selected at 12:01?
Why would this be more correct than locking the table and waiting until 1:00 and returning 5? Sure, this would be a long wait, but at 12:00 a command was issued to set every value to 5!

Answer 1

You are question is related to oracle transaction
control and read consistency in the database in
Oracle database all the SQL and DML runs with SCN
timestamp.
for example
Session 1 SQL 12:00 > UPDATE t1 SET c4=5 ;
---assuming this runs for an hour
before updating c4=5 it takes the snapshot of the C4
column data and keeps in
UNDO segments to make other sessions read
consistent.
Session 2 SQL 12:01 > SELECT c4 FROM t1 WHERE ....
this will return 4 as the DML is not yet comminted in
the Session 1
To happen this Oracle database server process reads
the data from UNDO to answer your question.locks are
matianed at the database BLOCK header level.
Let me put a simple thought here
You have two debit cards issued for one account one
is with you and the other one is with your beloved
one.Balance in your account = Rs 40000
1) at 12:00 you went to a ATM and inserted your debit
card to draw amt Rs 20000
and your transaction is still processing ( you not
recieved the cash)
2) at 12:01 your beloved one went an another ATM and
interested to see the balance so do you think Rs
40000 should be shown or Rs 20000.
hope you got some ideaNot to pick nits, but I think this is a bad example. If you had such an important action (especially if it was going to take a long time to complete) you would/should have some kind of lock on modifying the account value so that this very thing couldn't happen! Only one transaction at a time gets to update the balance. In a sense, that might make Oracle look like the SQL Server you appear to be familiar with (the 2nd transaction would wait. A very long time!) But for a bank account type transaction, you almost have to.
Gaff