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Ovid (2709)

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Stuff with the Perl Foundation. A couple of patches in the Perl core. A few CPAN modules. That about sums it up.

Journal of Ovid (2709)

Monday January 22, 2007
01:40 PM

MySQL Madness (ma, get my gun!)

[ #32232 ]

So I was debugging a MySQL problem and quickly hacked out the following query. It took a long time to run before finally giving me an 'OUT OF MEMORY' error:

SELECT distinct(ip)
FROM   foo
  SELECT distinct(ip)
  FROM   bar

See a problem with that query? The problem I have is one you can't see. A coworker pointed out a typo. There's a column, but not a bar.ip column. MySQL didn't catch that. It was apparently trying to run the outer query, collect all results and then parsing and filtering on the subquery.

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  • The folks at Oracle realized that antijoins were often performance bottlenecks and provided hints to specifically to speed them up, e.g. HASH_AJ, and specifying the optimizer type (RULE vs COST) can make a huge difference.

    I'm curious what the point of the 'distinct' in the subquery is, btw. I think that would make the explain plan worse in Oracle. But I digress.

    For stuff like this in Oracle, I generally opt for the simple minus query. Reasonable performance and easy to understand.

    How do the following

    • I couldn't really say how those others compare. I'm not a performance guru. I've traditionally gone more for the normalization aspect and not worried as much about the implementation, trusting the DBAs to be good at their jobs. Now I'm finding I need to focus on the implementation more and more.

      As for the 'distinct' clause in the subquery, I really thought there might be duplicate ip addresses in that table and I had no idea if MySQL would optimize those away or force every value to be compared multipl