You raise a good point here, MySQL also supports native JSON type since 5.7.8, and if you're using MySQL 8.* the support for JSON is even more robust.
I'd certainly consider a separate data store (if you want to isolate this data from your primary stores) in either PostgreSQL or MySQL with native JSON support unless there is some particular business case that rules these out (for the OP).
Our primary database is pretty much stuck on MySQL 5.6 for the foreseeable future due to boring reasons. You're right though in that I don't need this data in the same database (clearly, because it's not) so I could stand up a new MySQL 8 or PostgreSQL instance. I've been wanting to play with both of those anyway.
Thanks for the suggestion!
That's also a good option. If the primary DB is frozen in time and if this data is mostly written and not read, you could even have a separate MySQL connection with the newest version that only contains the table to store this data that gets updated asynchronously (read "eventually consistent").
You'd recycle the same tooling, company expertise and learn new things that might help in the future to upgrade the main DB if you ever chose to do so.
Adopting a completely new storage server is a big investment (in terms of money, devops, time and so on), I'd prioritise a way to decrease that impact if it's technologically feasible
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