This is said in Oracle7 Server Concepts Manual, Data Concurrency,              
Additional Considerations for Serializable Isolation:                          
                                                                               
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Both read committed and serializable transactions use row-level locking, and   
both will wait if they try to change a row updated by an uncommitted           
concurrent transaction. The second transaction that tries to update a given    
row waits for the other transaction to commit or rollback and release its      
lock. If that other transaction rolls back, the waiting transaction            
(regardless of its isolation mode) can proceed to change the previously        
locked row, as if the other transaction had not existed.                       
                                                                               
However, read committed and serializable transactions behave differently if    
the other (blocking) transaction commits. When the other transaction commits   
and releases its locks, a read committed transaction will proceed with its     
intended update...                                                             
^^^^^^^^                                                                       
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What does this mean? Will Oracle update this row (just updated by other        
Xaction)? In any case or only if qualification is ok for this row now 
(qual was ok for unchanged version of row but could be changed by
concurrent Xaction)?

Could someone run in Oracle test below?

1.  CREATE TABLE test (x integer, y integer)
2.  INSERT INTO test VALUES (1, 1);
    INSERT INTO test VALUES (1, 2);
    INSERT INTO test VALUES (3, 2);
3.  run two session T1 and T2 (in read committed mode)
4.  in session T2 run
    UPDATE test SET x = 1, y = 2 WHERE x <> 1 OR y <> 2;
5.  in session T1 run
    UPDATE test SET y = 3 WHERE x = 1;
6.  in session T2 run
    COMMIT;
7.  in session T1 run
    SELECT * FROM test;         -- results?
8.  in session T1 run
    COMMIT;
9.  now in session T2 run
    UPDATE test SET x = 2;
10. in session T1 run
    UPDATE test SET y = 4 WHERE x = 1;
11. in session T2 run
    COMMIT;
12. in session T1 run
    SELECT * FROM test;         -- results?

TIA,
	Vadim

Tue, 14 Jul 1998 19:04:14 +0800
Vadim Mikheev <vadim@krs.ru>
Q about read committed in Oracle...