Instead of trigger with Oracle sequence - oracle

Question

I have some old applications (legacy applications) which uses their own MAX NUMBER table. The problem is that we cannot modify and release all the applications at the same time. And therefore, we would like to do it in phases.
I came up with the following proposal. But, I do not know why Oracle executes the view's select query when I am executing the update statement.
Current usage:
-- Update the max number table
Update Sysadm.DummyTable
Set MaxNumber = MaxNumber + 1;
-- An application needs to select current value from the MaxNumber table.
Select * From Sysadm.DummyTable;
Proposed solution:
CREATE SEQUENCE SYSADM.SEQ_DummyTable START WITH 12345 INCREMENT BY 1;
--- Common function ---
--- The following function will get the current sequence number of the Oracle sequence across all the sessions.
CREATE OR REPLACE FUNCTION Sysadm.GetCurrValue(sLSeqName VARCHAR)
RETURN NUMBER
AS nLDummy NUMBER;
BEGIN
if (sLSeqName = 'SEQ_DUMMYTABLE') Then
SELECT SEQ_DUMMYTABLE.CURRVAL Into nLDummy FROM DUAL;
Else
Return null;
End If;
RETURN(nLDummy);
END;
/
--- The existing DummyTable table will be dropped and DummyTable (exactly with the same name including its structure) view will be created.
Create or Replace View Sysadm.DummyTable (MAXNUMBER)
As
Select sysadm.GetCurrValue('SEQ_DUMMYTABLE') As MAXNUMBER From Sysadm.ABC Where RowNum = 1;
--- The following trigger will get executed when the application tried to update the DummyTable table. This trigger will get executed instead of update statement from the application.
Create Or Replace Trigger Sysadm.TR_DummyTable
Instead of UPDATE ON Sysadm.DummyTable
Declare
nLDummy NUMBER;
Begin
SELECT SEQ_DUMMYTABLE.NEXTVAL Into nLDummy FROM DUAL;
End;
/
I have Instead of trigger on the view. However, When I am trying to execute the following update query I get an error message.
-- Update the max number table
Update Sysadm.DummyTable
Set MaxNumber = MaxNumber + 1;
ORA error message:
Error starting at line : 1 in command -
Update Sysadm.DUMMYTABLE Set MaxNumber = MaxNumber + 1
Error report -
SQL Error: ORA-08002: sequence SEQ_DUMMYTABLE.CURRVAL is not yet defined in this session
ORA-06512: at "SYSADM.GETCURRVALUE", line 10
08002. 00000 - "sequence %s.CURRVAL is not yet defined in this session"
*Cause: sequence CURRVAL has been selected before sequence NEXTVAL
*Action: select NEXTVAL from the sequence before selecting CURRVAL

Answer 1

As Oracle told you: you can't fetch currval if you didn't fetch from the sequence within this session, and you do that by selecting nextval. For example:
SQL> create sequence seqa;
Sequence created.
This doesn't work - you already know that, you hit the error already:
SQL> select seqa.currval from dual;
select seqa.currval from dual
*
ERROR at line 1:
ORA-08002: sequence SEQA.CURRVAL is not yet defined in this session
So: nextval first, currval next (if you need it):
SQL> select seqa.nextval from dual;
NEXTVAL
----------
1
SQL> select seqa.currval from dual;
CURRVAL
----------
1
SQL>
In your case, it means that this would look like
if (sLSeqName = 'SEQ_DUMMYTABLE') Then
SELECT SEQ_DUMMYTABLE.NEXTVAL Into nLDummy FROM DUAL;
Else -------

Answer 2

By just changing the function we can get it to work. This will fix the concurrency issue as well:
--- The following function will get the current sequence number of the Oracle sequence across all the sessions.
CREATE OR REPLACE FUNCTION Sysadm.GetCurrValue(sLSeqName VARCHAR)
RETURN NUMBER
AS nLDummy NUMBER;
BEGIN
if (sLSeqName = 'SEQ_DUMMYTABLE') Then
SELECT SEQ_DUMMYTABLE.CURRVAL Into nLDummy FROM DUAL;
Else
Return null;
End If;
RETURN(nLDummy);
EXCEPTION
WHEN OTHERS THEN
RETURN NULL;
END;
/