Does Parallel Hint within Dynamic SQL of Oracle run in parallel? - oracle

Question

I have both DML and DDL as part of my procedure and enabled the parallel on both DML and DDL. I want to run them in Parallel mode using parallel hint but neither of them execute in parallel. Is this a limitation of using the Dynamic SQL?
For example
DECLARE
v_parallel_degree NUMBER := 8;
BEGIN
EXECUTE IMMEDIATE 'ALTER SESSION FORCE PARALLEL DML PARALLEL ' || v_parallel_degree;
EXECUTE IMMEDIATE 'ALTER SESSION FORCE PARALLEL QUERY PARALLEL ' || v_parallel_degree;
EXECUTE IMMEDIATE 'ALTER SESSION FORCE PARALLEL DDL PARALLEL ' || v_parallel_degree;
EXECUTE IMMEDIATE 'INSERT /*+PARALLEL(DEFAULT)*/ INTO '|| p_target_tabname || ' NOLOGGING
SELECT /*+PARALLEL(dmf,DEFAULT)*/*
FROM ' || p_source_tabname ||' PARTITION('|| p_part_name ||');
EXECUTE IMMEDIATE 'CREATE UNIQUE INDEX idx_pk ON TAB_HIST
(COL1,COL2,COL3)
LOCAL
NOLOGGING PARALLEL ' || v_parallel_degree;
END;
I even tried the below block but not working.
v_sql := 'BEGIN
EXECUTE IMMEDIATE ''ALTER SESSION FORCE PARALLEL DML PARALLEL ' || v_parallel_degree ||''';
EXECUTE IMMEDIATE ''ALTER SESSION FORCE PARALLEL QUERY PARALLEL ' || v_parallel_degree ||''';
INSERT /*+PARALLEL(DEFAULT)*/ INTO '|| p_target_tabname || ' NOLOGGING
SELECT /*+PARALLEL(dmf,DEFAULT)*/*
FROM ' || p_source_tabname ||' PARTITION('|| p_part_name ||') dmf;
DBMS_OUTPUT.PUT_LINE(''Inserted '' || SQL%ROWCOUNT || '' Rows into Table- '' || p_target_tabname || '' Partition - '' || p_part_name );
COMMIT;
END;';
EXECUTE IMMEDIATE v_sql;
Oracle Version -
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
PL/SQL Release 12.1.0.2.0 - Production
CORE 12.1.0.2.0 Production
Soon will be upgraded to 19c.
Any suggestions are appreciated..
TIA
Venkat

Answer 1

TLDR
Most probably you forgot to enable parallel DML.
ALTER SESSION ENABLE PARALLEL DML;
Additionaly if you force parallel execution you typically do not use parallel hints and vice versa.
Sample Setup (11.2)
create table TAB_HIST (
col1 int,
col2 int,
col3 varchar2(4000))
PARTITION BY RANGE (col1)
interval(1000000)
(
partition p_init values less than (1000000)
);
create table TAB_SRC (
col1 int,
col2 int,
col3 varchar2(4000)
)
PARTITION BY RANGE (col1)
interval(1000000)
(
partition p_init values less than (1000000)
);
insert into tab_src
select rownum, rownum, rpad('x',1000,'y') from dual connect by level <= 100000;
commit;
Insert
You must enable parallel DML in the first step
ALTER SESSION ENABLE PARALLEL DML;
Note that alternatively a hint can be used
INSERT /*+ ENABLE_PARALLEL_DML */ …
Additionally if you force parallel DML and QUERY, you typically do not use parallel hints. I'm hinting a direct insert with APPEND that is often used in this situation.
DECLARE
v_parallel_degree NUMBER := 2;
BEGIN
EXECUTE IMMEDIATE 'ALTER SESSION FORCE PARALLEL DML PARALLEL ' || v_parallel_degree;
EXECUTE IMMEDIATE 'ALTER SESSION FORCE PARALLEL QUERY PARALLEL ' || v_parallel_degree;
EXECUTE IMMEDIATE 'INSERT /*+ APPEND */ INTO TAB_HIST
SELECT *
FROM TAB_SRC PARTITION(P_INIT)';
END;
/
How to check if the table was inserted in parallel?
The simplest way is to query the table (before making a commit) - if you get the bellow error, it way a parallel direct insert.
select count(*) from TAB_HIST;
ORA-12838: cannot read/modify an object after modifying it in parallel
Index
If you specify a parallel degree in the create index statement you need not enable or force anything.
DECLARE
v_parallel_degree NUMBER := 2;
BEGIN
EXECUTE IMMEDIATE 'CREATE UNIQUE INDEX idx_pk ON TAB_HIST
(COL1,COL2,COL3)
LOCAL
NOLOGGING PARALLEL ' || v_parallel_degree;
END;
/
The check is as simple as to look on the degree in data dictionary
select DEGREE from user_indexes where table_name = 'TAB_HIST';
DEGREE
---------
2
Note that after creating index in parallel mode you often want to reset the DOP to one. Otherwise some simple nested loop queries may be confused and will open a parallel query...

Answer 2

Is this a limitation of using the Dynamic SQL?
No.
may be helpful:
parallel DML : https://docs.oracle.com/database/121/VLDBG/GUID-1D5C8D6C-0A0E-4CDB-8B32-16EC3C856ACC.htm#VLDBG1431
restriction PDML: https://docs.oracle.com/database/121/VLDBG/GUID-6626C70C-876C-47A4-8C01-9B66574062D8.htm
parallel DDL: https://docs.oracle.com/database/121/VLDBG/GUID-41774038-773B-40A5-BDCD-AB16A189C035.htm#VLDBG1411

Answer 3

You can start with it: https://stackoverflow.com/a/67377464/429100
Then you can check a real execution plan (Note section) and RTSM (Real-time SQL Monitor) report (select/*+ no_monitor */ dbms_sqltune.report_sql_monitor(sql_id => '&1',report_level => 'ALL',type => 'TEXT') sqlmon from dual;). They should show more information about the used DOP.
And, finally, you can trace parallel execution using the following command:
alter session set "_px_trace"="compilation","execution","messaging";
More info: "Tracing Parallel Execution with _px_trace (Doc ID 444164.1)"