Rowscn strange behaviour - value decreased

Question

Hi,
We are identifying the changed data using rowscn. The source table is on a different database where a view is created with rowscn column from the table. From the target database, we select the rows from the source view where rowscn is greater than the previously checked max(rowscn). But we have noticed that sometimes the max(rowscn) selected is lesser than the value of previous max(rowscn). In which scenario, this could happen?
Regds

Answer 1

The max(rowscn) selected can be less than the value of previous max(rowscn) if the rows with the highest SCN have been deleted.
create table test (col1 varchar2(10), col2 varchar2(10)) rowdependencies;
insert into test (col1, col2) values ('A', '1');
commit;
insert into test (col1, col2) values ('B', '2');
commit;
select ora_rowscn, col1, col2 from test;
      ORA_ROWSCN COL1       COL2
  10908939964362 A          1
  10908939964364 B          2
2 rows selected.
select max(ora_rowscn) from test;
     MAX(ORA_ROWSCN)
      10908939964364
1 row selected.
delete from test where col1='B';
1 row deleted.
commit;
select max(ora_rowscn) from test;
     MAX(ORA_ROWSCN)
      10908939964362
1 row selected.
The new maximum 10908939964362 is 2 less than the old one 10908939964364.