Full sync in records

It is directly not possible because you need to communicate database through the Appian for utilizing the Record sync feature. Alternatively you can fetch all the data with batches and write them back to database. Add once a day you can sync the record type automatically. 

There is a way in my opinion to trigger a full sync for a record type when changes in the database occurs. Precondition is that you have access to the system/process which performs the database changes to introduce a Web API call.

In order to perform an ad-hoc sync of a record, a fake deployment must be performed with a properties fie referencing the relevant record uuids. This takes advantage of the following feature of Appian: Trigger a Sync by Fake Deployment
Sounds tricky, but it is working.

You need to use Sync Record Smart Service.

There are a few ways to sync your data using this smart service:

• Generate a web API that includes the a!syncRecords() function and add the API's URL to your source system's webhook so it can notify Appian of changes and sync them.
• Use the a!syncRecords() function in an interface so end users can choose to sync data whenever they submit a form or click a button.
• Use the Sync Records smart service in a process model to trigger a sync after a call to an integration or stored procedure executes, or to start a sync using a record action.

docs.appian.com/.../Sync_Records_Smart_Service.html