On the topic of plain text databases, I have been playing around with wordnet(basically the backbone data structure of a thesaurus ) and while there are a lot of perfectly good libraries to handle the data I was having fun building my own. One interesting thing is that the original data format has the byte offset of every record and link baked into the data, this makes it trivial to avoid having to load the whole thing into memory and you can directly seek to the record in question thus making it a plain text database. Admittedly one only good for reading as writes would have to rebuild all the indexes.
https://wordnet.princeton.edu/documentation/wndb5wn
Honestly now days the whole thing can be trivially loaded into memory but back when the project was started this was much more of a concern, I do know that once I figured this out I started re writing my program to see how little memory I could use, It was a lot of fun to use access patterns other than "load the whole thing into memory"
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