Sequence out of sync

Question

Database version : Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
There are thousands of schemas in our database. Those schemas are identical, and each of them have over 400 tables and 300 sequences. Most sequences have cache enabled, and cache size is usually 20.
As the number of schemas grow in the database, we frequently run into primary key constraint violation problems. The values of those primary key columns are from different sequences. If we check database when the problem occurs, sometimes we find the nextval of those sequences are smaller the max values of the corresponding columns already, but sometimes the sequences numbers are still greater than the max values of the columns.
The problem usually occurs when multiple processes are running schema change or data migration scripts concurrently. If we run one process at a time, the problem doesn't seem to happen.
Have anyone experienced similar issue or is there an Oracle bug related to it?
Thanks,

Answer 1

if we have multiple sessions running and each session is inserting records in different schema?Example :
Suppose i am running your application by scott user :
As you said each session is having their separate sequences and suppose scott.seq1.nextval is 123
I (scott) am going to insert into hr.tab1.col1
At the same time hr user is also running the application and suppose hr.seq1.nextval is 123
He (HR) is also going to insert into hr.tab1.col1
Now its PK violation, why ? Because I am using current user's sequence object i.e. in scott's session, I am using scott.seq1, while HR user is using hr.seq1; whose both sequence's nextval is 123. So, check the application code that by using whose sequence object you are going to insert into PK column; in the above example, you should use hr.seq1, because you are going to insert into hr.tab1.
Regards
Girish Sharma