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This java project gives you a glimpse at how it all works. It was extracted out of a now defunkt db vendor (I think?). https://calcite.apache.org/

Yeah, it came out of the Eigenbase project. (http://luciddb.sourceforge.net/ and http://www.eigenbase.org). DB internals are indeed quite fun. (I worked on LucidDB(https://github.com/LucidDB/luciddb) for a time.)