Why parallel to serial ?

Question

Hi,
Version 10204
I am trying to insert few millions of record from one partition table to another.
Both tables are declared as parallel .
select table_name,degree,instances from dba_tables where table_name in ('CI_CUST_INFO_BACKUP_ORIGINAL','CI_CUST_INFO_BACKUP');
TABLE_NAME                            DEGREE     INSTANCES
CI_CUST_INFO_BACKUP             8                        1
CI_CUST_INFO_BACKUP_ORIGINAL       DEFAULT        DEFAULTFor some reason the optimizer choose PARALLEL_TO_SERIAL and not PARALLEL_TO_PARALLEL .
I expect to see few sessions in v$sessions , but there is just one session running.
Can one suggest how to make it run in parallel ?
alter session force parallel dml;
Session altered.
alter session force parallel query;
Session altered.
INSERT      /*+ append */INTO acrm.ci_cust_info_backup
SELECT /*+ parallel (p,8) */      *
FROM acrm.ci_cust_info_backup_original PARTITION (cust_info_backup_def) p;
Plan
INSERT STATEMENT  ALL_ROWSCost: 12,135                                     
7 PX COORDINATOR                                
  6 PX SEND QC (RANDOM) PARALLEL_TO_SERIAL SYS.:TQ10001 :Q1001Cost: 12,135  Bytes: 7,069,977,612  Cardinality: 8,600,946                           
    5 LOAD AS SELECT PARALLEL_COMBINED_WITH_PARENT :Q1001                    
      4 PX RECEIVE PARALLEL_COMBINED_WITH_PARENT :Q1001Cost: 12,135  Bytes: 7,069,977,612  Cardinality: 8,600,946                 
        3 PX SEND RANDOM LOCAL PARALLEL_TO_PARALLEL SYS.:TQ10000 :Q1000Cost: 12,135  Bytes: 7,069,977,612  Cardinality: 8,600,946            
          2 PX BLOCK ITERATOR PARALLEL_COMBINED_WITH_CHILD :Q1000Cost: 12,135  Bytes: 7,069,977,612  Cardinality: 8,600,946  Partition #: 6  Partitions accessed #8     
            1 TABLE ACCESS FULL TABLE PARALLEL_COMBINED_WITH_PARENT ACRM.CI_CUST_INFO_BACKUP_ORIGINAL :Q1000Cost: 12,135  Bytes: 7,069,977,612  Cardinality: 8,600,946  Partition #: 7  Partitions accessed #8

Answer 1

Yoav wrote:
Hi,
Version 10204
I am trying to insert few millions of record from one partition table to another.
Both tables are declared as parallel .
For some reason the optimizer choose PARALLEL_TO_SERIAL and not PARALLEL_TO_PARALLEL .
I expect to see few sessions in v$sessions , but there is just one session running.
Can one suggest how to make it run in parallel ?Could you please post the formatted output taken from DBMS_XPLAN.DISPLAY, or even better DBMS_XPLAN.DISPLAY_CURSOR?
You need to understand that every plan at one point will have a parallel to serial - and the plan you've posted shows that the LOAD AS SELECT is run in parallel, and only the top-most parallel operation reports parallel to serial.
So from a plan perspective the execution should use parallel DML and parallel query.
Now at runtime there might be reasons why you don't get the degree you've requested so you should also check the session statistics for parallel executions downgraded and statements being executed in parallel:
select
       s.value
     , n.name
from
       v$statname n,
       v$sesstat s
where
       n.statistic# = s.statistic#
and    (n.name like '%downgrad%' or n.name like '%parallelized%')
and    s.sid = <qcsid>;Regards,
Randolf
Oracle related stuff blog:
http://oracle-randolf.blogspot.com/
Co-author of the "OakTable Expert Oracle Practices" book:
http://www.apress.com/book/view/1430226684
http://www.amazon.com/Expert-Oracle-Practices-Database-Administration/dp/1430226684