Wankun worked on the apache/spark repository, focusing on optimizing the performance of the substr SQL function in Apache Spark. By modifying the Java implementation, Wankun introduced logic to skip unnecessary calculations when the substring position is greater than zero, thereby reducing CPU usage for string-heavy SQL workloads. The approach preserved existing API behavior and ensured that functional semantics remained unchanged, with comprehensive updates to documentation and tests. Leveraging skills in Java, SQL, and performance optimization, Wankun’s work targeted improved query throughput and efficiency. The depth of the contribution lay in its careful integration and attention to maintaining backward compatibility.