Oqallisissiat Howard Miller-imit allatat

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From the comment in the code...

           // It's purpose is to locate all the active participants of a course with course completion enabled.
           // We also only want the users with no course_completions record as this functions job is to create
           // the missing ones smiler
           // We want to record the user's enrolment start time for the course. This gets tricky because there can be
           // multiple enrolment plugins active in a course, hence the possibility of multiple records for each
           // couse/user in the results.

Not sure if it helps.

It was added on the back of - https://moodle.atlassian.net/browse/MDL-65276 - which also doesn't help much

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There's no such thing as "database schema version" in Moodle - although it may not be exactly what you mean. The schema is constructed from instructions coded in every plugin. However, each plugin defines its own set of tables so this may not matter. If you go to Site administration > Plugins > Pugin overview, the 10-digit version number for every plugin defines the schema.

I'm not familiar enough with the Quiz / Question bank schema to answer the second part. Except that I am aware that it's probably more complex than you might anticipate.
 
I would probably have a look at ad-hoc_contributed_reports - there are definitely some SQL queries in there that extract quiz attempts. 
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Unfortunately, you're probably asking in the wrong place. We didn't create the Bitnami installer / upgrader, don't know how it works and what it might have got wrong. You probably need to ask Bitnami.

THIS... is why using these type of installers can be a bad idea. They're great until something goes wrong.