Performance problem with MERGE statement

Question

Version : 11.1.0.7.0
I have an insert statement like following which is taking less than 2 secs to complete and inserts around 4000 rows:
INSERT INTO sch.tab1
          (c1,c2,c3)
SELECT c1,c2,c3
   FROM sch1.tab1@dblink
  WHERE c1 IN (SELECT c1 FROM sch1.tab2@dblink);I wanted to change it to a MERGE statement just to avoid duplicate data. I changed it to following :
MERGE INTO sch.tab1 t1
USING (SELECT c1,c2,c3
   FROM sch1.tab1@dblink
  WHERE c1 IN (SELECT c1 FROM sch1.tab2@dblink) t2
ON (t1.c1 = t2.c1)
WHEN NOT MATCHED THEN
INSERT (t1.c1,t1.c2,t1.c3)
VALUES (t2.c1,t2.c2,t2.c3);The MERGE statement is taking more than 2 mins (and I stopped the execution after that). I removed the WHERE clause subquery inside the subquery of the USING section and it executed in 1 sec.
If I execute the same select statement with the WHERE clause outside the MERGE statement, it takes just 1 sec to return the data.
Is there any known issue with MERGE statement while implementing using above scenario?

Answer 1

riedelme wrote:
Are your join columns indexed?
Yes, the join columns are indexed.
You are doing a remote query inside the merge; remote queries can slow things down. Do you have to select all thr rows from the remote table? What if you copied them locally using a materialized view?Yes, I agree that remote queries will slow things down. But the same is not happening while select, insert and pl/sql. It happens only when we are using MERGE. I have to test what happens if we use a subquery refering to a local table or materialized view. Even if it works, I think there is still a problem with MERGE in case of remote subqueries (atleast till I test local queries). I wish some one can test similar scenarios so that we can know whether it is a genuine problem or some specific problem from my side.
>
BTW, I haven't had great luck with MERGE either :(. Last time I tried to use it I found it faster to use a loop with insert/update logic.
Edited by: riedelme on Jul 28, 2009 12:12 PM:) I used the same to overcome this situation. I think MERGE needs to be still improved functionally from Oracle side. I personally feel that it is one of the robust features to grace SQL or PL/SQL.