The Pega 7 Platform relational database holds the rules, data instances, work items, history, and other concrete objects from the internal classes of your Pega 7 Platform system. External classes, by definition, correspond to tables in other databases and are not part of the Pega 7 database.
Tables and views in the Pega 7 database become visible when you use Database Table instances (Data-Admin-DB-Table). For example, the Data-Admin-Operator-ID class defines users and their organization. The property pyUserName in that class can contain as its value a text string for a user name. When your system is first installed, instances of the Data-Admin-Operator-ID class are saved as rows in the pr_operators database table, which are identified in the database table instance named Data-Admin-Operator-ID. The column named pyUserName contains the text string for the name.
Your applications can use report definition rules or the Obj-Browse method to generate SQL statements that search the database and generate reports. For security and performance reasons, the Pega 7 Platform uses prepared statements to access the Pega 7 Platform database. The SELECT, WHEN, and other parts of the SQL statements are parameters of the prepared statements.
See the PDN article About PegaRULES table partitioning for information about dividing large work tables and indexes into smaller and more manageable tablespaces.