Clarification On Contention and Transaction

Question

Hi All;
I simply demonstrated Oracle's sequence mechanism own-made. I do not want ask about performance. My demonstration is below. When one of my friend see the code below, he asked me that in a multi-user environment, contention might occur and this contention would occur the same value for different processes. I had some tests but i could not get any problems. Updating and getting sequence is one transaction and function is automous.
Is the code below, can the same sequence can be generated?
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> drop table sequences;
Table dropped
SQL> create table sequences( name varchar2(32), current_value number, increment_value number );
Table created
SQL> alter table sequences add constraint pk_sequences primary key(name);
Table altered
SQL> insert into sequences values('T', 1, 1);
1 row inserted
SQL> commit;
Commit complete
SQL> CREATE OR REPLACE FUNCTION GetNextValue(pis_SequenceName VARCHAR2)
  2    RETURN NUMBER IS
  3    PRAGMA AUTONOMOUS_TRANSACTION;
  4    vn_NextValue NUMBER;
  5  BEGIN
  6    UPDATE sequences
  7       SET current_value = current_value + increment_value
  8     WHERE NAME = pis_SequenceName RETURNING current_value INTO vn_NextValue;
  9    COMMIT;
10    RETURN vn_NextValue;
11  END GetNextValue;
12  /
Function created
SQL> show err;
No errors for FUNCTION HR.GETNEXTVALUE
SQL> CREATE OR REPLACE FUNCTION GetCurrentValue(pis_SequenceName VARCHAR2)
  2    RETURN NUMBER IS
  3    vn_CurrentValue NUMBER;
  4  BEGIN
  5    SELECT current_value
  6      INTO vn_CurrentValue
  7      FROM sequences
  8     WHERE NAME = pis_SequenceName;
  9    RETURN vn_CurrentValue;
10  END GetCurrentValue;
11  /
Function created
SQL> create sequence seq_test_t
  2  start with 1
  3  increment by 1
  4  nocache;
Sequence created
SQL> SELECT GetNextValue('T') ud_next1
  2        ,GetCurrentValue('T') ud_curr1
  3        ,GetNextValue('T') ud_next2
  4        ,seq_test_t.NEXTVAL next1
  5        ,seq_test_t.CURRVAL curr1
  6        ,seq_test_t.NEXTVAL next2
  7    FROM dual;
  UD_NEXT1   UD_CURR1   UD_NEXT2      NEXT1      CURR1      NEXT2
         2          2          3          1          1          1
SQL> My tests are:
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> drop table t;
Table dropped
SQL> create table t(x varchar2(1), i number );
Table created
SQL> alter table t add constraint uk_i unique(i);
Table altered
SQL> SELECT * FROM t GROUP BY i HAVING COUNT(*) > 1;
SELECT * FROM t GROUP BY i HAVING COUNT(*) > 1
ORA-00979: not a GROUP BY expression
SQL> SELECT i FROM t GROUP BY i HAVING COUNT(*) > 1;
         I
SQL> SELECT count(*) from t;
  COUNT(*)
     50000
SQL>
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> set serverout on 1000
SQL> BEGIN
  2    DBMS_OUTPUT.PUT_LINE( 'Start A ' || dbms_utility.get_time );
  3    FOR i IN 1 .. 10000 LOOP
  4      INSERT INTO t VALUES ('A', GetNextValue('T'));
  5      COMMIT;
  6    END LOOP;
  7    DBMS_OUTPUT.PUT_LINE( 'End  A ' || dbms_utility.get_time );
  8  END;
  9  /
Start A 9904878
End  A 9906156
PL/SQL procedure successfully completed
SQL>
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> set serverout on 1000
SQL> BEGIN
  2    DBMS_OUTPUT.PUT_LINE( 'Start B ' || dbms_utility.get_time );
  3    FOR i IN 1 .. 10000 LOOP
  4      INSERT INTO t VALUES ('B', GetNextValue('T'));
  5      COMMIT;
  6    END LOOP;
  7    DBMS_OUTPUT.PUT_LINE( 'End B ' || dbms_utility.get_time );
  8  END;
  9  /
Start B 9904928
End B 9906018
PL/SQL procedure successfully completed
SQL>
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> set serverout on 1000
SQL> BEGIN
  2    DBMS_OUTPUT.PUT_LINE( 'Start C ' || dbms_utility.get_time );
  3    FOR i IN 1 .. 10000 LOOP
  4      INSERT INTO t VALUES ('C', GetNextValue('T'));
  5      COMMIT;
  6    END LOOP;
  7    DBMS_OUTPUT.PUT_LINE( 'End C ' || dbms_utility.get_time );
  8  END;
  9  /
Start C 9904982
End C 9906104
PL/SQL procedure successfully completed
SQL>
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> set serverout on 1000
SQL> BEGIN
  2    DBMS_OUTPUT.PUT_LINE( 'Start D ' || dbms_utility.get_time );
  3    FOR i IN 1 .. 10000 LOOP
  4      INSERT INTO t VALUES ('D', GetNextValue('T'));
  5      COMMIT;
  6    END LOOP;
  7    DBMS_OUTPUT.PUT_LINE( 'End D ' || dbms_utility.get_time );
  8  END;
  9  /
Start D 9905045
End D 9905768
PL/SQL procedure successfully completed
SQL>
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as hr
SQL>
SQL> set serverout on 1000
SQL> BEGIN
  2    DBMS_OUTPUT.PUT_LINE( 'Start E' || dbms_utility.get_time );
  3    FOR i IN 1 .. 10000 LOOP
  4      INSERT INTO t VALUES ('E', GetNextValue('T'));
  5      COMMIT;
  6    END LOOP;
  7    DBMS_OUTPUT.PUT_LINE( 'End E ' || dbms_utility.get_time );
  8  END;
  9  /
Start E9905109
End E 9906189
PL/SQL procedure successfully completed
SQL>
A Start     B Start     C Start     D Start     E Start                                         D End     C End           A End           B End           E End
9904878     9904928     9904982     9905045     9905109      "All 5 proceeses are working here"  9905768     9906104     9906156     9906156     9906189

Answer 1

Since you're serializing the sequence generation through an autonomous transaction with an UPDATE statement, I don't see how it should be possible to generate duplicates. Ask your friend to provide a test case, I'd say.
C.