ORACLE - insert records by multiple user on PK table - oracle

Question

In oracle I have a table say EMPLOYEE(EMPID,EMPNAME) with primary key on EMPID.
Two users are having insert privileges to this table say USER1 and USER2.
User1 insert a record with EMPID=1 and EMPNAME='XYZ' and not committed. But if USER2 is trying to insert the same record EMPID=1 and EMPNAME='XYZ' then the screen get hangs till user1 commits or rollback.
Is there option to insert this record by both the users with out any hang and the user who commits second should get the PK violation error.
Thanks,
Niju Jose

Answer 1

In a single-user database, the user can modify data in the database without concern for other users modifying the same data at the same time. However, in a multiuser database, the statements within multiple simultaneous transactions can update the same data. Transactions executing at the same time need to produce meaningful and consistent results.
Isolation Level Dirty Read Nonrepeatable Read Phantom Read
---------------- ------------ ------------------ -------------
Read uncommitted Possible Possible Possible
Read committed Not possible Possible Possible
Repeatable read Not possible Not possible Possible
Serializable Not possible Not possible Not possible
In your case, Oracle acquires a row level lock. The insert case is simpler, because inserted rows are locked, but also are not seen by other users because they are not commited. When the user commits, he also releases the locks, so, other users can view these rows, update them, or delete them.
Read here about Data concurrency and consistency.
Also see here more details on Insert Mechanism
To explain more, I reproduce Florin's answer here
For example, let be tableA(col1 number, col2 number), with this data within it:
col1 | col2
1 | 10
2 | 20
3 | 30
If user John issues at time1:
update tableA set col2=11 where col1=1;
will lock row1.
At time2 user Mark issue an
update tableA set col2=22 where col1=2;
the update will work, because the row 2 is not locked.
Now the table looks in database:
col1 | col2
1 | 11 --locked by john
2 | 22 --locked by mark
3 | 30
For Mark table is(he does not see the changes uncommited)
col1 | col2
1 | 10
2 | 22
3 | 30
For John table is:(he does not see the changes uncommited)
col1 | col2
1 | 11
2 | 20
3 | 30
If mark tries at time3:
update tableA set col2=12 where col1=1;
his session will hang until time4 when John will issue an commit.(Rollback will also unlock the rows, but changes will be lost)
table is(in db, at time4):
col1 | col2
1 | 11
2 | 22 --locked by mark
3 | 30
Immediatley, after John's commit, the row1 is unlocked and marks's update will do the job:
col1 | col2
1 | 12 --locked by mark
2 | 22 --locked by mark
3 | 30
lets's mark issue a rollbak at time5:
col1 | col2
1 | 11
2 | 20
3 | 30
So your hang is due to the fact that the Oracle engine is waiting for a commit or a rollback from User1, before giving a definitive response to user2.