Due to the limitations of the contract, the external system cannot be read, and more data needs to be stored in the actual business to execute the contract based on the data. And these data are currently stored in the chainbase memory database. The limitation is that the cost is very high, and the space is very limited, so can you save the data to an external system, such as MongoDB, MySQL? These professional storage systems must be less expensive than chainbases, and have a larger storage capacity, and performance is sufficient under normal circumstances.
Yes, you can save parts of the data you need in MongoDB or MySQL.
The library to enable that is called demux-js.
There is a template and demo explaining how to use that library here.
One important issue you need to keep in mind, any data that you don't save in RAM is not accessible to the smart contract code. So you can't have business logic in the smart contract that depends on data in the external database.