arnipetursson (11/12/2013)
When I run the two queries (outer join vs NOT EXISTS),the outer join query runs in 4-9ms (69 logical IO n the temp tables and 0 scans and 0 logical IO on the work table)
and the not exists query runs in 28-32ms (109 logical IO on the temp tables and 50 scans and 5079 logical IO on the work table).
It looks like the #CourseCompleted table is scanned 50 times in the NOT EXISTS query, whereas only 30 times in the outer join query.
I wonder if these results hold over larger data sets.
I usually use a LEFT JOIN for this reason.
deroby (11/12/2013)
2. I've seen it 'fail' when people use a <somefield> that turns out to be NULL-able
That is easily avoidable, particularly when you're joining on a foreign key, by choosing <somefield> as one of the columns in your JOIN criteria. I agree though that it is a possible newbie mistake.
Nice article Patrick!
My thought question: Have you ever been told that your query runs too fast?
My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.
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Since random numbers are too important to be left to chance, let's generate some![/url]
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