Convenience of the complex queries. If you need anything more than a simple key-value access, you'll in fact need to implement the query engine yourself if you decide to use NoSQL.
Note that the author specifically mentioned that they use HandlerSocket for fast key retrievals and SQL for complex queries, and both methods work together on the very same database.
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What do you actually get from using MySQL?
Convenience of the complex queries. If you need anything more than a simple key-value access, you'll in fact need to implement the query engine yourself if you decide to use NoSQL.
Note that the author specifically mentioned that they use HandlerSocket for fast key retrievals and SQL for complex queries, and both methods work together on the very same database.