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18 March, 2015

Parallel Execution -- 2c PX Servers

Adding to my two previous posts here and here about identifying usage of Parallel Execution -- exactly how many PX servers were used for a query, here is a third method.  (the first two are V$PX_PROCESS/V$PX_SESSION and V$SQLSTATS.PX_SERVERS_EXECUTIONS).  This method uses V$PQ_SESSTAT.

However, the limitation of V$PQ_SESSTAT is that it can only be queried from the same session as that which ran the Parallel Query.  The other two methods can be used by a separate "monitoring" session.

HEMANT>show parameter parallel_max

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
parallel_max_servers                 integer     8
HEMANT>connect / as sysdba
Connected.
SYS>alter system set parallel_max_servers=64; 

System altered.

SYS>show parameter cpu

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

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
parallel_degree_policy               string      MANUAL
SYS>



As has been identified earlier, the PARALLEL Hint will use 16 PX Servers (limited by PARALLEL_MAX_SERVERS [see this post] because of the values of CPU_COUNT and PARALLEL_THREADS_PER_CPU (where, in this case, PARALLEL_DEGREE_POLICY is yet MANUAL).

SYS>alter system flush shared_pool;

System altered.

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

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

HEMANT>select * from v$pq_sesstat;

STATISTIC                      LAST_QUERY SESSION_TOTAL
------------------------------ ---------- -------------
Queries Parallelized                    1             1
DML Parallelized                        0             0
DDL Parallelized                        0             0
DFO Trees                               1             1
Server Threads                         16             0
Allocation Height                      16             0
Allocation Width                        1             0
Local Msgs Sent                       464           464
Distr Msgs Sent                         0             0
Local Msgs Recv'd                     464           464
Distr Msgs Recv'd                       0             0

11 rows selected.

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

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

HEMANT>select * from v$pq_sesstat;

STATISTIC                      LAST_QUERY SESSION_TOTAL
------------------------------ ---------- -------------
Queries Parallelized                    1             2
DML Parallelized                        0             0
DDL Parallelized                        0             0
DFO Trees                               1             2
Server Threads                         16             0
Allocation Height                      16             0
Allocation Width                        1             0
Local Msgs Sent                       464           928
Distr Msgs Sent                         0             0
Local Msgs Recv'd                     464           928
Distr Msgs Recv'd                       0             0

11 rows selected.

HEMANT>

As we can see, the SESSIONS_TOTAL count of Server Threads does not get updated (although the count of Queries Parallelized is updated).  This behaviour remains in 12c.  (However, there are two additional statistics available in 12c).
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