Parallel Execution -- 2b PX Servers

Continuing my previous post, here I demonstrate  using V$SQLSTATS.PX_SERVERS_EXECUTIONS and a couple of issues around it.

I have restarted the database.

[oracle@localhost ~]$ sqlplus hemant/hemant

SQL*Plus: Release 11.2.0.2.0 Production on Fri Mar 13 22:49:20 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

HEMANT>show parameter cpu

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cpu_count                            integer     4
parallel_threads_per_cpu             integer     4
resource_manager_cpu_allocation      integer     4
HEMANT>select degree from user_tables where table_name = 'LARGE_TABLE';

DEGREE
----------------------------------------
         1

HEMANT>select /*+ PARALLEL */ count(*) from Large_Table;

  COUNT(*)
----------
   4802944

HEMANT>select executions, px_servers_executions, sql_fulltext
  2  from v$sqlstats
  3  where sql_id = '8b0ybuspqu0mm';

EXECUTIONS PX_SERVERS_EXECUTIONS SQL_FULLTEXT
---------- --------------------- --------------------------------------------------------------------------------
         1                    16 select /*+ PARALLEL */ count(*) from Large_Table

HEMANT>
HEMANT>select /*+ PARALLEL */ count(*) from Large_Table;

  COUNT(*)
----------
   4802944

HEMANT>select executions, px_servers_executions, sql_fulltext
  2  from v$sqlstats
  3  where sql_id = '8b0ybuspqu0mm';

EXECUTIONS PX_SERVERS_EXECUTIONS SQL_FULLTEXT
---------- --------------------- --------------------------------------------------------------------------------
         2                    32 select /*+ PARALLEL */ count(*) from Large_Table

HEMANT>

[Note : To understand why the executions took 16 PX Servers inspite of the degree on table being 1, see this post]
So we see that PX_SERVERS_EXECUTIONS shows cumulative statistics.  Let's try a slight twist.

HEMANT>connect / as sysdba
Connected.
SYS>alter system set parallel_max_servers=8;

System altered.

SYS>connect hemant/hemant
Connected.
HEMANT>select /*+ PARALLEL */ count(*) from Large_Table;

  COUNT(*)
----------
   4802944

HEMANT>select executions, px_servers_executions, sql_fulltext
  2  from v$sqlstats
  3  where sql_id = '8b0ybuspqu0mm';

EXECUTIONS PX_SERVERS_EXECUTIONS SQL_FULLTEXT
---------- --------------------- --------------------------------------------------------------------------------
         3                    40 select /*+ PARALLEL */ count(*) from Large_Table

HEMANT>

Because I set PARALLEL_MAX_SERVERS to 8, my query on Large_Table could take only 8 PX Servers at the next execution.  V$SQLSTATS.PX_SERVERS_EXECUTIONS now shows a cumulative count of 40 for 3 executions. There is no way to determine how many PX Servers were used in each of the 3 executions, because the history of executions is not maintained.
(In my controlled experiment, we know, by deduction, that the 3rd execution took 8 PX Servers simply because we know already that the first 2 executions took a cumulative count of 32 PX Servers -- by deducting 32 from 40 to get 8 for the 3rd execution)

What happens if the SQL is invalidated ?

HEMANT>alter table large_table parallel 4;

Table altered.

HEMANT>select /*+ PARALLEL */ count(*) from Large_Table;

  COUNT(*)
----------
   4802944

HEMANT>select executions, px_servers_executions, sql_fulltext
  2  from v$sqlstats
  3  where sql_id = '8b0ybuspqu0mm';

EXECUTIONS PX_SERVERS_EXECUTIONS SQL_FULLTEXT
---------- --------------------- --------------------------------------------------------------------------------
         1                     8 select /*+ PARALLEL */ count(*) from Large_Table

HEMANT>

The ALTER TABLE, being a DDL, had invalidated the query on Large_Table.  So, V$SQLSTATS also got reset.  Therefore, EXECUTIONS reset to 1 and PX_SERVES_EXECUTIONS got reset to 8.

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Parallel Execution -- 2 PX Servers

I've posted a couple of examples (here and here) of Parallel Execution servers for Parallel Query.

How do we identify usage of Parallel Execution ?

I will update this post (and, possibly, subsequent post(s)) with a few methods.

The first one (as I've shown in my previous posts) is to look at the column PX_SERVERS_EXECUTIONS in either V$SQLSTATS or V$SQL.  This can identify the number of PX Servers used for an SQL (Query or DML).  However, there is a caveat when the SQL undergoes multiple execution -- the statistic on PX_SERVERS_EXECUTIONS may be cumulative (i.e. additive) across all the executions of the SQL.  UPDATE 13-Mar-15 : See the new post here.

Another method is to look at the V$PX_PROCESS and V$PX_SESSION views.

Let me demonstrate this second method using the same SQL query from my previous blog post.

[oracle@localhost ~]$ sqlplus hemant/hemant

SQL*Plus: Release 11.2.0.2.0 Production on Tue Mar 3 23:34:37 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

HEMANT>select distinct sid from v$mystat;

       SID
----------
       197

HEMANT>select count(*) from v$px_process;

  COUNT(*)
----------
         0

HEMANT>select count(*) from v$px_session;

  COUNT(*)
----------
         0

HEMANT>select degree from user_tables where table_name = 'LARGE_TABLE';

DEGREE
----------------------------------------
         1

HEMANT>show parameter cpu 

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cpu_count                            integer     4
parallel_threads_per_cpu             integer     4
resource_manager_cpu_allocation      integer     4
HEMANT>set serveroutput off
HEMANT>select /*+ PARALLEL */ count(*) from Large_Table;

  COUNT(*)
----------
   4802944

HEMANT>select count(*) from v$px_process;

  COUNT(*)
----------
        16

SYS>select qcsid, req_degree, degree, count(*)
  2  from v$px_session
  3  group by qcsid, req_degree, degree
  4  /

     QCSID REQ_DEGREE     DEGREE   COUNT(*)
---------- ---------- ---------- ----------
       197                                1
       197         16         16         16

SYS>

The query by the SYS user is from a different session while the "select /*+ PARALLEL */ count(*) from Large_Table;" is being executed by HEMANT.  This query is on V$PX_SESSION and shows only when the Parallel Query sessions are active -- i.e. running HEMANT's  parallel count(*) query.  (If I query V$PX_SESSION after the parallel count(*) completes, I won't get the information).

The above output demonstrates
(a) that there were no PX servers before I began the parallel count(*) query and there were 16 at the end -- 16 PX servers had been started and had not yet shutdown by the time I queried V$PX_PROCESS (They will shutdown after a while  ** note below).
(b) that my parallel count(*) query (executed by SID 197 which is the QueryCo-ordinator -- represented by QCSID) DID request and use 16 PX server sessions (as evidenced in the output from the query on V$PX_SESSION).  Thus, what I claimed on the basis of PX_SERVERS_EXECUTION in my previous post is correct.

** Note : A few minutes later, I can see that the PX Servers have shutdown.

HEMANT>select count(*) from v$px_process
  2  /

  COUNT(*)
----------
         0

HEMANT>

Later, I will demonstrate how to join V$PX_PROCESS and V$PX_SESSION.
I will also demonstrate how you manage the number of PX Servers.

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Parallel Execution -- 1b The PARALLEL Hint and AutoDoP (contd)

Continuing the previous thread, having restarted the database again, with the same CPU_COUNT and missing I/O Calibration statistics  ....

The question this time is : What if the table level DoP is specifically 1 ?

[oracle@localhost ~]$ sqlplus '/ as sysdba'

SQL*Plus: Release 11.2.0.2.0 Production on Mon Mar 2 23:22:28 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SYS>show parameter cpu

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cpu_count                            integer     4
parallel_threads_per_cpu             integer     2
resource_manager_cpu_allocation      integer     4
SYS>show parameter parallel

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
fast_start_parallel_rollback         string      LOW
parallel_adaptive_multi_user         boolean     FALSE
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     135
parallel_min_percent                 integer     0
parallel_min_servers                 integer     0
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
SYS>

SYS>select * from dba_rsrc_io_calibrate;

no rows selected

SYS>
SYS>connect hemant/hemant
Connected.
HEMANT>set serveroutput off
HEMANT>select degree from user_tables where table_name = 'LARGE_TABLE';

DEGREE
----------------------------------------
         4

HEMANT>alter table large_table parallel 1;

Table altered.

HEMANT>select degree from user_tables where table_name = 'LARGE_TABLE';

DEGREE
----------------------------------------
         1

HEMANT>select /*+ PARALLEL */ count(*) from LARGE_TABLE;

  COUNT(*)
----------
   4802944

HEMANT>select * from table(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  47m7qyrj6uzqn, child number 0
-------------------------------------
select /*+ PARALLEL */ count(*) from LARGE_TABLE

Plan hash value: 2085386270

-----------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name        | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |             |       |  2622 (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 |             |  4802K|  2622   (1)| 00:00:32 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS FULL| LARGE_TABLE |  4802K|  2622   (1)| 00:00:32 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------

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

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

Note
-----
   - automatic DOP: skipped because of IO calibrate statistics are missing


27 rows selected.

HEMANT>select px_servers_executions from v$sqlstats where sql_id='47m7qyrj6uzqn';

PX_SERVERS_EXECUTIONS
---------------------
                    8

HEMANT>

Aaha ! Again ! The same SQL statement, the same SQL_ID, the same Execution Plan (Plan Hash Value) and Oracle chose to use 8 PX Servers for the query !  Again, ignoring the table level DoP (of 1)

So, once again, we see that Oracle actually computes a DoP that looks like it is CPU_COUNT x PARALLEL_THREADS_PER_CPU. Let's verify this.

HEMANT>connect / as sysdba
Connected.
SYS>alter system set parallel_threads_per_cpu=4;

System altered.

SYS>alter system flush shared_pool;

System altered.

SYS>connect hemant/hemant
Connected.
HEMANT>select degree from user_tables where table_name = 'LARGE_TABLE';

DEGREE
----------------------------------------
         1

HEMANT>set serveroutput off                        
HEMANT>select /*+ PARALLEL */ count(*) from Large_Table;

  COUNT(*)
----------
   4802944

HEMANT>select * from table(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  8b0ybuspqu0mm, child number 0
-------------------------------------
select /*+ PARALLEL */ count(*) from Large_Table

Plan hash value: 2085386270

-----------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name        | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |             |       |  1311 (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 |             |  4802K|  1311   (1)| 00:00:16 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS FULL| LARGE_TABLE |  4802K|  1311   (1)| 00:00:16 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------

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

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

Note
-----
   - automatic DOP: skipped because of IO calibrate statistics are missing


27 rows selected.

HEMANT>select px_servers_executions from v$sqlstats where sql_id='8b0ybuspqu0mm';

PX_SERVERS_EXECUTIONS
---------------------
                   16

HEMANT>

YES SIR ! Oracle chose to use 16 PX Servers this time. So that does look like CPU_COUNT x PARALLEL_THREADS_PER_CPU.  Have you also noticed the COST ?  The COST has also dropped to half.  So, the COST is also computed based on the number of PX Servers that it expects to be able to grab and use.

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