Oracle Tunning in 10g on table parallelism

Question

Hi,
I am working in 10g database(10.2.0.1). I have insert statement which uses table A(120 M rows), B(0.5M rows) and C (0.2M rows) to insert on table D.
Following are the table definitions:
Table A - Is partitioned, has unique indexe and is declared NOPARALLEL DEGREE.
Table B - Has unique indexe and is declared NOPARALLEL DEGREE.
Table C - Has unique indexe and is declared NOPARALLEL DEGREE.
Table D - Has declared NOPARALLEL DEGREE.
a) When using the insert on table D using tables A,B and C using HINT /*+ APPEND PARALLEL(table name 12) / and also given on Inset table with /+ APPEND PARALLEL(table name 12). For most of the time it shows 17 to 21 sessions.
INSERT INTO /*+ APPEND PARALLEL(d 12)*/ D d
(x,y,z)
SELECT /*+ PARALLEL(A 12) PARALLEL(B 12) PARALLEL(C 12)*/
bla. bla...bla...
with few joins on the respective tables
b) Same query without any changes at table level definition and no query changes. Some time it shows only one session. At that particular time we (user) has to intervene and alter table definition from NOPARALLEL to PARALLEL (DEGREE DEFAULT) on target table D.
My questions are:
i) What could be reason that it varies and displays no of sessions are running ?
ii) How oracle takes and uses no of sessions while executing the query?
iii) Is there any where we can see or monitor the session tracking ?
iv) How many max sessions can a query like above insert can take/use for the user schema?
v) Is it possible for the developers to reduce and increase the sessions while executing the query ?
vi) On what basis the sessions get increase or decrease?
If any of them can provide answers to above query it will be helpful...
Regards,
Prakash C

Answer 1

The following link might help u a bit in understanding parallelism concept for queries -
http://docs.oracle.com/cd/B19306_01/server.102/b14223/usingpe.htm