V$BGPROCESS and V$SESSION

Question

This is on 10g.
V$BGPROCESS displays information about background processes
V$SESSION has some rows representing background processes.
What is the difference between two ?
Am I correct in understanding that ; v$bgprocess contains total allocated processes whereas, v$session represents total runtime processes on that instance?
That being said, for example, if there are 10 sessions, all 10 sessions will have same number of bacground processes. Is that true?
To explain further, say if v$session displays DBW0 and DBW1, it means two instance of DBWR processes are running for this instance. But if v$bgprocess has DBW0.. DBW9 processes, that means, you can maximum have 9 DBWR processes on that instance.
Please concur if above is correct.
Thanks
R
t

Answer 1

it gives 157 rows, but in production it gives only 70 rows. Does this mean that maximum number of bg processes allowed are less on prod box than dev box?Your production database has 70 BG process, so its showing you this value.
The BG process can differ instance to instance, such as ARCH process, if archiving is enable for an instance, then you will have this one from your query, and you will not have this one where the archiving is not enabled.
You can set the PROCESSES parameter in DB, there is no parameter relate to BG process solely.
you would have the value of PADDR=00 for the inactive BG processes.
Those BG processes are active will have a value of PADDR which is the address of the process state object.
Look at below :
you will find that PADDR !='00' from v$bgprocess retrieve the same columns as querying the v$process and v$session by joining and have the PADDR=value/not 00, that is the active BG processes
SQL> ed
Wrote file afiedt.buf
1 select a.name,a.paddr,b.program,b.status,b.user#,b.username
2 from v$bgprocess a
3 join v$session b
4* on (a.paddr = b.paddr)
SQL> /
NAME PADDR PROGRAM STATUS USER# USERNAME
MMNL 2ECF1944 ORACLE.EXE (MMNL) ACTIVE 0
MMON 2ECF14C4 ORACLE.EXE (MMON) ACTIVE 0
QMNC 2ECF0744 ORACLE.EXE (QMNC) ACTIVE 0
CJQ0 2ECEF0C4 ORACLE.EXE (CJQ0) ACTIVE 0
RECO 2ECEEC44 ORACLE.EXE (RECO) ACTIVE 0
SMON 2ECEE7C4 ORACLE.EXE (SMON) ACTIVE 0
CKPT 2ECEE344 ORACLE.EXE (CKPT) ACTIVE 0
LGWR 2ECEDEC4 ORACLE.EXE (LGWR) ACTIVE 0
DBW0 2ECEDA44 ORACLE.EXE (DBW0) ACTIVE 0
MMAN 2ECED5C4 ORACLE.EXE (MMAN) ACTIVE 0
PMON 2ECED144 ORACLE.EXE (PMON) ACTIVE 0
11 rows selected.
SQL> select * from v$bgprocess where paddr!=00;
select * from v$bgprocess where paddr!=00
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected BINARY got NUMBER
SQL> select * from v$bgprocess where paddr!='00';
PADDR PSERIAL# NAME DESCRIPTION ERROR
2ECED144 1 PMON process cleanup ##########
2ECED5C4 1 MMAN Memory Manager ##########
2ECEDA44 1 DBW0 db writer process 0 ##########
2ECEDEC4 1 LGWR Redo etc. ##########
2ECEE344 1 CKPT checkpoint ##########
2ECEE7C4 1 SMON System Monitor Process ##########
2ECEEC44 1 RECO distributed recovery ##########
2ECEF0C4 1 CJQ0 Job Queue Coordinator ##########
2ECF0744 1 QMNC AQ Coordinator ##########
2ECF14C4 1 MMON Manageability Monitor Process ##########
2ECF1944 1 MMNL Manageability Monitor Process 2 ##########
11 rows selected.