Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 14 ■ PARALLEL EXECUTION 629
We’ll start by creating the USER_INFO table, enabling it for parallel operations, and then
gathering statistics on it:
big_table@ORA10GR1> create table user_info as select * from all_users;
Table created.
big_table@ORA10GR1> alter table user_info parallel;
Table altered.
big_table@ORA10GR1> exec dbms_stats.gather_table_stats( user, 'USER_INFO' );
PL/SQL procedure successfully completed.
Now, we would like to parallel direct path load a new table with this information. The
query we’ll use is simply
create table new_table parallel
as
select a.*, b.user_id, b.created user_created
from big_table a, user_info b
where a.owner = b.username
The plan for that particular CREATE TABLE AS SELECT looked like this in Oracle 10g:
---------------------------------------------------------------------------
| Id | Operation | Name | TQ |IN-OUT| PQ Distrib |
---------------------------------------------------------------------------
| 0 | CREATE TABLE STATEMENT | | | | |
| 1 | PX COORDINATOR | | | | |
| 2 | PX SEND QC (RANDOM) | :TQ10001 | Q1,01 | P->S | QC (RAND) |
| 3 | LOAD AS SELECT | | Q1,01 | PCWP | |
|* 4 | HASH JOIN | | Q1,01 | PCWP | |
| 5 | PX RECEIVE | | Q1,01 | PCWP | |
| 6 | PX SEND BROADCAST | :TQ10000 | Q1,00 | P->P | BROADCAST |
| 7 | PX BLOCK ITERATOR | | Q1,00 | PCWC | |
| 8 | TABLE ACCESS FULL| USER_INFO | Q1,00 | PCWP | |
| 9 | PX BLOCK ITERATOR | | Q1,01 | PCWC | |
| 10 | TABLE ACCESS FULL | BIG_TABLE | Q1,01 | PCWP | |
---------------------------------------------------------------------------
If you look at the steps from 4 on down, that is the query (SELECT) component. The scan
of BIG_TABLE and hash join to USER_INFO was performed in parallel, and each of the subresults
was loaded into a portion of the table (step 3, the LOAD AS SELECT). After each of the parallel
execution servers finishes its part of the join and load, it sends its results up to the query coordinator.
In this case, the results simply indicated “success” or “failure,” as the work had already
been performed.
And that is all there is to it—parallel direct path loads made easy. The most important
thing to consider with these operations is how space is used (or not used). Of particular
importance is a side effect called extent trimming. I’d like to spend some time investigating
that now.