Enabling parallellism in oracle sql

Question

Hi All,
I have a question related to parallilism enabling statements .
I have seen that before running a sql statement we may use
alter session enable parallel DDL;
alter session enable parallel query;
alter session enable parallel DML;
What exactly these statement mean ??
If i mention parallel hints like below statement ..
select /*+ parallel(a,8) */ count(1) from customer a;
parallelism won't happen Until session is altered with above statement ??
Is thr any depedency b/w above 'alter session' statement and parallel hint in query .
Thanks

Answer 1

By default parallel execution is enabled for DDL and query statements. Unless you had turn it off somewhere then you need to manually enable it subsequentially.
Overview of Parallel Execution
http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/manproc.htm#sthref688
for example, I login to my DB without issue enable statement.
%sqlplus test/test
SQL*Plus: Release 10.1.0.2.0 - Production on Tue Sep 1 12:15:26 2009
Copyright (c) 1982, 2004, Oracle.  All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options
TEST@etest> set autotrace on
TEST@etest> select /*+ parallel(a,8) */ count(1) from test a
  2  /
  COUNT(1)
         4
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=0 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     PX COORDINATOR
   3    2       PX SEND* (QC (RANDOM)) OF ':TQ10000'                   :Q1000
   4    3         SORT* (AGGREGATE)                                    :Q1000
   5    4           PX BLOCK* (ITERATOR) (Cost=0 Card=4)               :Q1000
   6    5             TABLE ACCESS* (FULL) OF 'TEST' (TABLE) (Cost=0 C :Q1000
          ard=4)
   3 PARALLEL_TO_SERIAL
   4 PARALLEL_COMBINED_WITH_PARENT
   5 PARALLEL_COMBINED_WITH_CHILD
   6 PARALLEL_COMBINED_WITH_PARENT