Why doesn't a parallel query in parallel?

Hello

I hope someone can help me.

I tested (several times and different ways) to see if the parallel server will work and help on my queries.

My version is 11.2 on Solaris 10.

I have a table with more 100,000 records and set autotrace and calendar on for testing purposes.

I confirmed PARALLEL_MIN_SERVERS = 30 and PARALLEL_MAX_SERVERS = 160 and PARRALLEL_SERVERS_PER_CPU = 2.
(my server has 4 CPU)

I tested the full table running scan (select * from table) and time taken about 13:30 minutes.

Then, I changed the table and set 4 Parallels.
Run the test again and he ran more slowly (about 16 minutes).

Then I ran select / * + parallel (manual) * / and it was always about 14 minutes.

Then I ran select / * + parallel 4 * / and it took about 14:30 minutes.

Then I reset the degree of parallel to the table to 1.
Then, ran select / * + parallel 4 * / and it took a little less than 13:30 minutes.

So, it seems that it does not use any parallel query. Why not?
SQL> sho parameter cpu

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ---------
cpu_count                            integer                          4
parallel_threads_per_cpu             integer                          2
resource_manager_cpu_allocation      integer                          4

SQL> sho parameter parallel

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ---------
fast_start_parallel_rollback         string                           LOW
parallel_adaptive_multi_user         boolean                          TRUE
parallel_automatic_tuning            boolean                          FALSE
parallel_degree_limit                string                           CPU
parallel_degree_policy               string                           MANUAL
parallel_execution_message_size      integer                          16384
parallel_force_local                 boolean                          FALSE
parallel_instance_group              string
parallel_io_cap_enabled              boolean                          FALSE
parallel_max_servers                 integer                          160
parallel_min_percent                 integer                          0
parallel_min_servers                 integer                          30
parallel_min_time_threshold          string                           AUTO
parallel_server                      boolean                          FALSE
parallel_server_instances            integer                          1
parallel_servers_target              integer                          64
parallel_threads_per_cpu             integer                          2
recovery_parallelism                 integer                          0

SQL> sho parameter servers

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- ---------
max_shared_servers                   integer
parallel_max_servers                 integer                          160
parallel_min_servers                 integer                          30
parallel_servers_target              integer                          64
shared_servers                       integer                          1

PX COORDINATOR stage is something that you will see that when the plan uses parallel queries that are steps PX SEND QC (RANDOM) and PX BLOCK ITERATOR. You will also see that data in the column of PQ Distrib if parallel query is used. And the reference to the column of the IN-OUT backwards-> operations S indicates a transition between the parallel operations to the series (i.e. several parallel slaves, aggregated by one master series).

Justin

Tags: Database

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