How many Mandatory Process are there in Postgres

Hi All,

When we start the postgres server, the writer process, wal process,
postmaster, autovacuum ( if autovacuum is on), stats collector will come
into picture as mandotory process. My question is, is there any processes
apart from these process, what are the mandotory process come along with the
postgres server and how many sleeping processes are there.

Few important question about Postgres Architecture
======================================
1. When does temp_buffer comes into existence in a database session ( like
when shared_buffer completely filled or any wal_buffer filled) ?
2. What is process array in shared memory?
3. maintenance_work_mem is used for vacuuming(does this memory allocated if
autovacuum is off)?
4. As oracle, Postgres doesnt have any undo_tablespace, for rollback
transaction. But it is handled in BEGIN /END block. So question is where the
transaction data is stored.

Thanks in Advance

Regards
Raghavendra