Have separate schemas for data and rules helps in an upgrade by allowing you to upgrade each schema separately. Because of the way Pega upgrades work, this lets you do an upgrade without having to shut down Pega. This enables several options for in place upgrades, including (using other HA tools) a seamless upgrade with zero downtime.
The Pega Platform supports a split schema database architecture. The split separates rules and data into separate schemas and is essential for enabling minimal to zero down time during Pega Platform application and strategic application upgrades or patch installation. The use of split schema architecture enables Pega Platform System Administrators to install and upgrade a new rules schema in the production database while the old schema is still in use. Pega Platform high availability features can then be used to migrate users from the old rule schema to the new schema on the production system to complete the upgrade.