I have a merge query of below format –
merge into dest using (select /*+ use_hash(t1,t2) parallel (4)*/ t1_name ,count(*) from table1 , table2 on t1.col=t2.col group by t1_name) src on (values) when matched then update dest.col
now I have to do union in this query
merge into dest using (select count(*),t1_name from (select /*+ use_hash(t1,t2) parallel (4)*/ t1_name ,count(*) from table1 , table2 on t1.col=t2.col group by t1_name union all select t1_name,count(*) from table t1 group by t1_name from table t1 group by t1_name ) group by t1_name) src on (values) when matched then update dest.col
My question is do I have to give parallel hint in second union clause also . Also use_hash hint which was running good earlier would not have impact on adding union clause in the query
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Answer
The hints apply to each select independently. First, you need to understand how parallel execution works in terms of session level and/or hints applied.
In this you have three elements
- The merge statement itself which will update some rows and will not run in parallel
- The first data set recover with the first select will run in parallel
- The second will run in parallel because of the hints applied to the first statement
I am assuming the tables are configured with noparallel and that you are not enabling parallel dml. I won’t even try to replicate the query, because I don’t know exactly what you want to do with it.
Test case
SQL> create table t1 ( c1 number, c2 number ) ;
Table created.
SQL> create table t2 ( c1 number, c3 number ) ;
Table created.
SQL> ed
Wrote file afiedt.buf
1 declare
2 begin
3 for i in 1 .. 10000
4 loop
5 insert into t1 values ( i , dbms_random.value );
6 insert into t2 values ( i , dbms_random.value );
7 end loop;
8* end;
SQL> /
PL/SQL procedure successfully completed.
SQL> ed
Wrote file afiedt.buf
1 declare
2 begin
3 for i in 1 .. 10000
4 loop
5 insert into t1 values ( i , dbms_random.value );
6 end loop;
7* end;
SQL> /
PL/SQL procedure successfully completed.
SQL> select count(*) from t1 ;
COUNT(*)
----------
20000
SQL> select count(*) from t2 ;
COUNT(*)
----------
10000
Now I am going to build a merge statement to update the table t2 using a source based on a union select. Forget in this case the meaning of the query, which has no sense at all, but the resulting execution plan
SQL> merge into t dest
2 using ( select distinct t1 from (
3 select /*+ use_hash(t2,t1) parallel (4)*/ t1.c1 as t1 from t1 inner join t2 on ( t1.c1=t2.c1)
4 union all
5 select t2.c1 as t1 from t1 inner join t2 on ( t1.c1=t2.c1 )
6 )) src on ( src.t1 = dest.c1 )
7* when matched then update set dest.c2 = dbms_random.value
10000 rows merged.
Execution Plan
----------------------------------------------------------
Plan hash value: 889987475
-----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------------------------
| 0 | MERGE STATEMENT | | 10000 | 253K| 43 (3)| 00:00:01 | | | |
| 1 | MERGE | T | | | | | | | |
| 2 | PX COORDINATOR | | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10006 | 10000 | 498K| 43 (3)| 00:00:01 | Q1,06 | P->S | QC (RAND) |
| 4 | VIEW | | | | | | Q1,06 | PCWP | |
|* 5 | HASH JOIN | | 10000 | 498K| 43 (3)| 00:00:01 | Q1,06 | PCWP | |
| 6 | PX RECEIVE | | 10000 | 126K| 17 (6)| 00:00:01 | Q1,06 | PCWP | |
| 7 | PX SEND BROADCAST | :TQ10005 | 10000 | 126K| 17 (6)| 00:00:01 | Q1,05 | P->P | BROADCAST |
| 8 | VIEW | | 10000 | 126K| 17 (6)| 00:00:01 | Q1,05 | PCWP | |
| 9 | HASH UNIQUE | | 10000 | 126K| 17 (6)| 00:00:01 | Q1,05 | PCWP | |
| 10 | PX RECEIVE | | 10000 | 126K| 17 (6)| 00:00:01 | Q1,05 | PCWP | |
| 11 | PX SEND HASH | :TQ10004 | 10000 | 126K| 17 (6)| 00:00:01 | Q1,04 | P->P | HASH |
| 12 | HASH UNIQUE | | 10000 | 126K| 17 (6)| 00:00:01 | Q1,04 | PCWP | |
| 13 | VIEW | | 40000 | 507K| 16 (0)| 00:00:01 | Q1,04 | PCWP | |
| 14 | UNION-ALL | | | | | | Q1,04 | PCWP | |
|* 15 | HASH JOIN | | 20000 | 507K| 8 (0)| 00:00:01 | Q1,04 | PCWP | |
| 16 | PX RECEIVE | | 10000 | 126K| 3 (0)| 00:00:01 | Q1,04 | PCWP | |
| 17 | PX SEND HASH | :TQ10000 | 10000 | 126K| 3 (0)| 00:00:01 | Q1,00 | P->P | HASH |
| 18 | PX BLOCK ITERATOR | | 10000 | 126K| 3 (0)| 00:00:01 | Q1,00 | PCWC | |
| 19 | TABLE ACCESS FULL| T2 | 10000 | 126K| 3 (0)| 00:00:01 | Q1,00 | PCWP | |
| 20 | PX RECEIVE | | 20000 | 253K| 5 (0)| 00:00:01 | Q1,04 | PCWP | |
| 21 | PX SEND HASH | :TQ10001 | 20000 | 253K| 5 (0)| 00:00:01 | Q1,01 | P->P | HASH |
| 22 | PX BLOCK ITERATOR | | 20000 | 253K| 5 (0)| 00:00:01 | Q1,01 | PCWC | |
| 23 | TABLE ACCESS FULL| T1 | 20000 | 253K| 5 (0)| 00:00:01 | Q1,01 | PCWP | |
|* 24 | HASH JOIN | | 20000 | 507K| 8 (0)| 00:00:01 | Q1,04 | PCWP | |
| 25 | PX RECEIVE | | 10000 | 126K| 3 (0)| 00:00:01 | Q1,04 | PCWP | |
| 26 | PX SEND HASH | :TQ10002 | 10000 | 126K| 3 (0)| 00:00:01 | Q1,02 | P->P | HASH |
| 27 | PX BLOCK ITERATOR | | 10000 | 126K| 3 (0)| 00:00:01 | Q1,02 | PCWC | |
| 28 | TABLE ACCESS FULL| T2 | 10000 | 126K| 3 (0)| 00:00:01 | Q1,02 | PCWP | |
| 29 | PX RECEIVE | | 20000 | 253K| 5 (0)| 00:00:01 | Q1,04 | PCWP | |
| 30 | PX SEND HASH | :TQ10003 | 20000 | 253K| 5 (0)| 00:00:01 | Q1,03 | P->P | HASH |
| 31 | PX BLOCK ITERATOR | | 20000 | 253K| 5 (0)| 00:00:01 | Q1,03 | PCWC | |
| 32 | TABLE ACCESS FULL| T1 | 20000 | 253K| 5 (0)| 00:00:01 | Q1,03 | PCWP | |
| 33 | PX BLOCK ITERATOR | | 94911 | 3522K| 26 (0)| 00:00:01 | Q1,06 | PCWC | |
| 34 | TABLE ACCESS FULL | T | 94911 | 3522K| 26 (0)| 00:00:01 | Q1,06 | PCWP | |
-----------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("SRC"."T1"="DEST"."C1")
15 - access("T1"."C1"="T2"."C1")
24 - access("T1"."C1"="T2"."C1")
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
- Degree of Parallelism is 4 because of hint
- PDML is disabled in current session
The most important parts of the execution plan below:
- As the tables has no statistics, Oracle has used dynamic sampling.
- The degree used in 4 due to the hing.
- Parallel DML is not enabled at session level, therefore the update resulting of the merge runs in noparallel.
- Oracle builds an VIEW dynamically to join the two datasets of the union, as a result of one of them is running in parallel, the CBO runs in parallel the second one even though it has no hints.
- In a USE_HASH hint use always as driving table the smaller table. Normally the CBO will always do that, so I would recommend to carefully use the USE_HASH hint, because if the smaller table grows to a point that is even greater than the second one, your hint will produce a lot of performance degradation.
Hope it clarifies.