When dealing with large data volumes in Salesforce and ensuring optimal performance for queries, it's important to choose an approach that both handles the data volume efficiently and retrieves the complete result set.

Option D is correct because using the Database.QueryLocator method is a best practice for handling large data volumes. It is typically used with batch Apex, which can process records in batches, thus reducing the likelihood of hitting governor limits. It's designed to handle very large data sets that would otherwise exceed normal SOQL query limits.

Option A is incorrect because creating a formula field doesn't improve the performance of the query. It simply creates a new field that combines existing data, but it does not inherently optimize the query execution.

Option B is incorrect because breaking down the query into two parts and joining them in Apex could potentially be less efficient and would require additional code to manage the combined result sets. This approach does not leverage the built-in Salesforce features designed to handle large data volumes.

Option C is incorrect because the @Future annotation makes the method execute asynchronously, but it does not help with query performance or large data volume management.