Apex method deleteImmediate() deletes data in a custom big object. Declare an sObject that contains all the fields in the custom big object’s index. The sObject acts like a template. All rows that match the sObject’s fields and values are deleted.
deleteImmediate(): This method initiates requests to delete the external data that corresponds to the specified external object records. The requests are executed synchronously and are sent to the external systems that are defined by the external objects’ associated external data sources.