Hey Venkateshkumar,
I had simillar requirements as you mentioned, instead of 200 attributes it's was 100 and instead million records it was half a millions and also based on that import I create custom soft search and display it on some complicated way as client wanted, it's was fairly easy to build it and Mendix can handle it without any issue. And your seconds question 2 - Mendix database will accommodate that amount of data in In memory. It's depends how you will build logic you need to understand how GC work in Mendix every end activity in Mendix is bassically triggering GC to clean-up memory so if you build it based on Mendix best practice you will have no issue and pretty good performance. And additionally to this
Majorly am looking to understand is Mendix is having in memory database. we can use it as cache only right.
Shall we store some value as persistent.
I'm pretty sure in all other programming language if you try to keep millions of records in memory, you will hit the memory leak at some point so don't do that, yes you should store it as persistent entites.
If you have any more question be free to add as comment to this answer.
Best regards, Slavko