Migration of million rows from remote table using merge

Question

I need to migrate (using merge) almost 15 million rows from a remote database table having unique index (DAY, Key) with the following data setup.
DAY1 -- Key1 -- NKey11
DAY1 -- Key2 -- NKey12
DAY2 -- Key1 -- NKey21
DAY2 -- Key2 -- NKey22
DAY3 -- Key1 -- NKey31
DAY3 -- Key2 -- NKey32
In my database, I have to merge all these 15 million rows into a table having unique index (Key); no DAY in destination table.
First, it would be executed for DAY1 and the merge command will insert following two rows. For DAY2, it would update two rows with the NKey2 (for each Key) values and so on.
Key1 -- NKey11
Key2 -- NKey12
I am looking for the best possible approach. Please note that I cannot make any change at remote database.
Right now, I am using the following one which is taking huge time for DAY2 and so on (mainly update).
MERGE INTO destination D
  USING (SELECT /*+ DRIVING_SITE(A) */ DAY, Key, NKey
               FROM source@dblink A WHERE DAY = v_day) S
  ON (D.Key = S.Key)
WHEN MATCHED THEN
   UPDATE SET D.NKey = S.NKey
WHEN NOT MATCHED THEN
   INSERT (D.Key, D.NKey) VALUES (S.Key, S.NKey)
LOG ERRORS INTO err$_destination REJECT LIMIT UNLIMITED;Edited by: 986517 on Feb 14, 2013 3:29 PM
Edited by: 986517 on Feb 14, 2013 3:33 PM

Answer 1

MERGE INTO destination D
  USING (SELECT /*+ DRIVING_SITE(A) */ DAY, Key, NKey
               FROM source@dblink A WHERE DAY = v_day) S
  ON (D.Key = S.Key)
WHEN MATCHED THEN
   UPDATE SET D.NKey = S.NKey
WHEN NOT MATCHED THEN
   INSERT (D.Key, D.NKey) VALUES (S.Key, S.NKey)
LOG ERRORS INTO err$_destination REJECT LIMIT UNLIMITED;The first remark I have to emphasize here is that the hint /*+ DRIVING_SITE(A) */ is silently ignored because in case of insert/update/delete/merge the driving site is always the site where the insert/update/delete is done.
http://jonathanlewis.wordpress.com/2008/12/05/distributed-dml/#more-809
Right now, I am using the following one which is taking huge time for DAY2 and so on (mainly update).The second remark is that you've realised that your MERGE is taking time but you didn't trace it to see where time is being spent. For that you can either use the 10046 trace event or at a first step get the execution plan followed by your MERGE statement.
LOG ERRORS INTO err$_destination REJECT LIMIT UNLIMITED;The third remark is related to the DML error logging : be aware that unique keys will empeach the DML error loggig to work correctly.
http://hourim.wordpress.com/?s=DML+error
And finally I advise you to look at the following blog article I wrote about enhancing an insert/select over db-link
http://hourim.wordpress.com/?s=insert+select
Mohamed Houri
www.hourim.wordpress.com