Kerry Osborne's Oracle Blog

The Oracle 11g parameter parallel_degree_limit is designed to put a cap on the maximum DOP for a statement.

The Oracle Database Reference 11g Release 2 (11.2) says this:

A numeric value for this parameter specifies the maximum degree of parallelism the optimizer can choose for a SQL statement when automatic degree of parallelism is active. Automatic degree of parallelism is only enabled if PARALLEL_DEGREE_POLICY is set to AUTO or LIMITED.

But that’s not entirely correct because it turns out you can enable auto DOP via a hint. The PARALLEL hint without a valid object on which to act will enable auto DOP for the statement. Here is an example:

SYS@DEMO1> @parms
Enter value for parameter: parallel_degree
Enter value for isset: 
Enter value for show_hidden: 

NAME                                               VALUE                                                                  ISDEFAUL ISMODIFIED ISSET
-------------------------------------------------- ---------------------------------------------------------------------- -------- ---------- ----------
parallel_degree_limit                              CPU                                                                    FALSE    TRUE       TRUE
parallel_degree_policy                             MANUAL                                                                 FALSE    TRUE       TRUE

Elapsed: 00:00:00.00
SYS@DEMO1> select /*+ parallel */ count(*) from eo00.SALES_1M2;

  COUNT(*)
----------
   1000000

Elapsed: 00:00:00.02
SYS@DEMO1> @x

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  aszs6rg3fttrt, child number 2
-------------------------------------
select /*+ parallel */ count(*) from eo00.SALES_1M2

Plan hash value: 3155295854

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name      | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |           |       |    25 (100)|          |        |      |            |
|   1 |  SORT AGGREGATE                |           |     1 |            |          |        |      |            |
|   2 |   PX COORDINATOR               |           |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM)         | :TQ10000  |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE             |           |     1 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR         |           |   982K|    25   (4)| 00:00:01 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS STORAGE FULL| SALES_1M2 |   982K|    25   (4)| 00:00:01 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   6 - storage(:Z>=:Z AND :Z<=:Z)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 7


28 rows selected.

Elapsed: 00:00:00.02

So as you can see, the hint enabled auto DOP, even though the parallel_degree_policy is set to manual. So let's see if the parallel_degree_limit will kick in for such a case.

SYS@DEMO1> alter session set parallel_degree_limit=4;

Session altered.

Elapsed: 00:00:00.00
SYS@DEMO1> select /*+ parallel */ count(*) from eo00.SALES_1M2;

  COUNT(*)
----------
   1000000

Elapsed: 00:00:00.02
SYS@DEMO1> @x

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  aszs6rg3fttrt, child number 1
-------------------------------------
select /*+ parallel */ count(*) from eo00.SALES_1M2

Plan hash value: 3155295854

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name      | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |           |       |    44 (100)|          |        |      |            |
|   1 |  SORT AGGREGATE                |           |     1 |            |          |        |      |            |
|   2 |   PX COORDINATOR               |           |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM)         | :TQ10000  |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE             |           |     1 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR         |           |   982K|    44   (3)| 00:00:01 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS STORAGE FULL| SALES_1M2 |   982K|    44   (3)| 00:00:01 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   6 - storage(:Z>=:Z AND :Z<=:Z)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 4 because of degree limit


28 rows selected.

Elapsed: 00:00:00.02

Yes it does. So what does it mean? Well for one thing the documentation (and some presentations I have seen recently) are slightly wrong. But the real moral of the story is that just because you have parallel_degree_policy set to manual, doesn't mean you are not using auto DOP.

Does this mean that the other features enabled by auto DOP (parallel statement queuing and in-memory parallel) will kick in on these kinds of statements as well? I'll leave that as an exercise for the reader.