Client has a database with millions of records on teacher and public employee salaries over the past 10 years for Ohio. They want to redesign their website and move it to WordPress -- any suggestions?
It's not clear from your question, but I'm assuming your client wants a frontend built in WordPress for this data. So assuming you'd be building a custom plugin.
Depending on the data structure, it may be best to keep this data in its own tables - within the same WP database if you like.
You can indeed create custom post types in WordPress and store any kind of data. But you may hit performance bottlenecks when you want to perform searches or want data in particular format.
We've built and tested some of our plugins with millions of rows of data. When we need higher performance, we've used our own tables inside WP. Rather than relying on WP's core data structures.
Hope this helps. And I'd be happy to answer any specific questions you may have.