MySQL has a lot of references to SPJ in MySQL including in official Oracle slides but I can't find what it actually stands for. I see it show up in a few blogs too.
I looked in the source and found nothing.
What does SPJ stand for?
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Apparently it stands for "Select Project Join":
What’s in a name?
If some of this seems familiar but you don’t remember hearing the term “Adaptive Query Localization” before then you’re not going crazy – previous internal names were SPJ (Select Project Join) and Pushed-Down Joins. We just figured that Adaptive Query Localization was more descriptive.