See https://pdn.pega.com/reporting-overview/reporting-overview and the section on "How does PRPC store and access data?"

The Platform uses the BLOB to store case and work data, enhancing performance and flexibility in operations on individual cases or work items. However, reports (and other list-centric activities such as displaying work lists and work queues, or finding open items by Customer or Policy ID) retrieve not the entire contents of the BLOB, but only specific properties from multiple cases or work items through database queries using the SQL language. PRPC reports generate appropriate SQL queries automatically for the user, based on the user’s description of the desired report. Retrieving specific properties directly from the BLOB for many instances can be inefficient. Because of this, PRPC offers a hybrid data storage model in which data can be stored as relational database columns as well as in the BLOB, offering the best of both approaches.  This should satisfy most of your reporting needs where the reports only include data from the Pega DB.

If you want to export some or all of the data from the PRPC database to an external data warehouse, perhaps to combine it with data from other systems of record to provide enterprise-wide reporting to management, use Pegasystems’ Business Intelligence Exchange (BIX) product to extract the desired data from the PRPC database.