I haven’t posted anything for a while, but that doesn’t mean that nothing was happening around MySQL Optimizer. In fact, exactly the reverse was true – so many thing were happening that I didn’t have time to blog.
I’ll try to make up for the lost coverage. Let’s start from smaller things. On Nov, 1 I’ve had a MySQL University session titled How MySQL Handles ORDER BY, GROUP BY, and DISTINCT.
The session page has slides and audio. If you’re ready to spend 1 hour on MySQL’s ways of doing ordering, you can listen to the audio. Otherwise, you can just scroll through the slides. They are reasonably self-contained and explain MySQL’s ORDER BY and GROUP BY handling in easy-to-digest colorful pictures.
Sergei Petrunia's blog 
What about ORDER BY NULL ? It turns off certain sorts? Perhaps even the “ORDER BY group_by_list is guaranteed for free” (slide 12)?
Won’t GROUP BY sometimes use a hash, thereby have to do extra work to guarantee ORDER BY?
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