Duplicate key exception on merge statement in DB2

问题

The problem: Everyday we get lots of parts that we want to add to our stock. We get messages over a queue that we read from (using 4 different servers). The queue always contains elements so the servers read as fast as they can. We want the servers to simply update the article if the article exits, and insert it if it doesn't.

Our first, naive solution was simply to select to see if the article existed, and if it didn't we wanted to insert. However since there was no row for us to lock we got problems with two servers doing the select at the same time, finding nothing, and then trying to insert. Of course one of them gave us a duplicate key exception.

So instead we looked to the merge statement. We made a merge statement that looked like this (simplified for clarity):

    MERGE INTO articles sr
    USING ( 
        VALUES (:PARAM_ARTICLE_NUMBER))
        AS v(ARTICLE_NUMBER)
    ON sr.ARTICLE_NUMBER = v.ARTICLE_NUMBER
    WHEN MATCHED THEN 
        UPDATE SET 
        QUANTITY = QUANTITY + :PARAM_QUANTITY
                ARRIVED_DATE = CASE WHEN ARRIVED_DATE IS NULL
                THEN :PARAM_ARRIVED_DATE
                ELSE ARRIVED_DATE END
    WHEN NOT MATCHED THEN
        INSERT (QUANTITY, ARRIVED_DATE)
        VALUES (:PARAM_QUANTITY, CURRENT_TIMESTAMP);

However, for some reason we are still getting duplicate key problems. My believe is that even if the merge statement is atomic two merge statements can run concurrently and select at the same time.

Is there any way, short of locking the whole table, to make sure we only get one insert?

回答1:

In a similar situation running the MERGE with the Repeatable Read isolation level solved our problem. RS was insufficient, because it still allowed phantom rows, which is exactly the issue you are experiencing. You can simply add WITH RR at the end of the statement and try it out.

Our test suite runs with up to 1000 simultaneous connections and we don't see concurrency much affected by the RR isolation used for that particular statement only.

回答2:

Do the insert first, catch the duplicate key exception if thrown; then update instead.

Charles

来源：https://stackoverflow.com/questions/34652886/duplicate-key-exception-on-merge-statement-in-db2