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SELECT NULL FROM DUAL FOR UPDATE NOWAIT

From: Sandra Becker <sbecker6925_at_gmail.com>
Date: Tue, 3 Jul 2007 08:07:47 -0600
Message-ID: <3c5f7820707030707v5d17b374p9e0860f390d78896@mail.gmail.com>


RHEL4, 9.2.0.8 Last Tuesday night we put some new code into production. The performance has been slower than usual, but no one seems to believe that it was the new code. Then Friday morning when I was out of the office and unreachable, performance tanked so badly they assumed the database had crashed. The only thing my SAs thought to do was make sure the database was still up. Still having no backup, no one trapped SQL, looked at the database, etc. Fortunately, I have been trapping information from sar and vmstat as well as some database session and process information. Based on the data I trapped and statspack reports, it seems the OS was thrashing for about 40 minutes. I found a couple of really, really bad SQL (4M+ buffer gets each) that were run only during that period on that Friday morning for the past 32 days. I don't have statspack data any older than that. I also trap FTS info and found the following statement in my FTS log:

     SELECT NULL FROM DUAL FOR UPDATE NOWAIT; No one is owning up to writing this code and I can prove it came from the application. I've never seen this before and wondered why anyone would do this. I do know that this statement was executed near the beginning of the time that things started to get out of control, but it wasn't the initial cause. The two bad SQL came first.

Questions:
1) Could this statement have contributed to the overall problems? 2) Is this something I should concern myself with or would I be spinning my wheels?
3) Why would anyone write a statement like this? What would be the point? Since I don't know which developer wrote it and no one is owning up to it, I haven't been able to find out what the desired result was supposed to be.

I appreciate any comments/suggestions.

Sandy

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Received on Tue Jul 03 2007 - 09:07:47 CDT

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