Parallel query for UPDATE

Question

Hi,
In the documentation of Oracle 10.2 -> Data Warehousing guide -> Ch.25 Using Parallel execution, search for section labeled "Rules for UPDATE, MERGE and DELETE", it is mentioned that,
"UPDATE, MERGE, and DELETE operations are parallelized by partition or subpartition. Update, merge, and delete parallelism are not possible within a partition, nor on a nonpartitioned table"
I am on 10.2 Enterprise edition, Solaris. I can see that, I can parallelize an UPDATE command (commands pasted below).
Please can somebody explain me, what am I missing ??
SQL> drop table r1 ;
Table dropped.
SQL> create table r1 as select * from user_objects where 1=2 ;
Table created.
SQL> exec dbms_stats.set_table_stats( ownname=>'solarp', tabname=>'R1', numrows=> 100000000, numblks=>251342 ) ;
PL/SQL procedure successfully completed.
SQL> alter table r1 parallel 5 ;
Table altered.
SQL> set autotrace traceonly explain
SQL> update r1 set status='XX' where created > ( sysdate - 7) ;
0 rows updated.
Execution Plan
Plan hash value: 1840129185
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
| 0 | UPDATE STATEMENT | | 5000K| 66M| 14083 (14)| 00:02:49 | | | |
| 1 | UPDATE | R1 | | | | | | | |
| 2 | PX COORDINATOR | | | | | | | | |
| 3 | PX SEND QC (RANDOM)| :TQ10000 | 5000K| 66M| 14083 (14)| 00:02:49 | Q1,00 | P->S | QC (RAND) |
| 4 | PX BLOCK ITERATOR | | 5000K| 66M| 14083 (14)| 00:02:49 | Q1,00 | PCWC | |
|* 5 | TABLE ACCESS FULL| R1 | 5000K| 66M| 14083 (14)| 00:02:49 | Q1,00 | PCWP | |
Predicate Information (identified by operation id):
5 - filter("CREATED">SYSDATE@!-7)
Thanks in advance

Answer 1

The manuals get out of date and out of synch. Somewhere in the 9i manuals it says that there is "limited" support for all parallel DML. Your plan, as David said, is a parallel select with serial update - if you want to perform the update in parallel then you need an explicit:
alter session enable parallel dml;Here's a plan (pulled from v$sql_plan in 10.2.0.3) for a similar update to yours. This plan is the parallel update version:
update t1 set n1 = 0 where mod(id,100) = 0
| Id  | Operation             | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
|   0 | UPDATE STATEMENT      |          |       |       |   753 (100)|          |        |      |            |
|   1 |  PX COORDINATOR       |          |       |       |            |          |        |      |            |
|   2 |   PX SEND QC (RANDOM) | :TQ10000 |   100 |   800 |   753   (0)| 00:00:10 |  Q1,00 | P->S | QC (RAND)  |
|   3 |    UPDATE             | T1       |       |       |            |          |  Q1,00 | PCWP |            |
|   4 |     PX BLOCK ITERATOR |          |   100 |   800 |   753   (0)| 00:00:10 |  Q1,00 | PCWC |            |
|*  5 |      TABLE ACCESS FULL| T1       |   100 |   800 |   753   (0)| 00:00:10 |  Q1,00 | PCWP |            |
Predicate Information (identified by operation id):
   5 - access(:Z>=:Z AND :Z<=:Z)
       filter(MOD("ID",100)=0)Note, particularly, that the UPDATE operation is the child of the PX SEND to coordinator. i.e. we update in the PX slaves, then tell the co-ordinator what we updated.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk