David Gelhar optimized principal-based session retrieval in the mmoayyed/cas repository by redesigning how the MongoDB ticket registry queries sessions. He replaced an inefficient text index search with a direct IDX_PRINCIPAL index lookup for the getSessionsFor(principalId) operation, which reduced query latency and resource consumption. David also removed the obsolete JSON text index, simplifying database maintenance and paving the way for future optimizations. To ensure reliability, he added targeted tests for session retrieval and deletion by principal. His work demonstrated depth in backend development, database management, and Java, resulting in a more maintainable and performant session management subsystem.