Developed an expression-level tracing feature for the facebookincubator/velox repository, enabling detailed tracking of output batches from named expression functions such as UDFs. The work involved refactoring trace lifecycle management by introducing scoped initialization and per-expression tracer ownership, implemented in C++ to minimize runtime overhead and reduce maintenance risk. Tracers were managed using unique pointers and selective activation within operators marked for tracing, ensuring efficient performance. The implementation included comprehensive end-to-end validation and test coverage, with hot-path checks optimized using FOLLY_UNLIKELY. This enhancement established a foundation for deeper debugging and performance profiling in Velox’s query execution framework.