Worked on the apache/spark repository to address a complex bug in Spark SQL’s handling of nested Common Table Expressions (CTEs) under unions. Focused on improving the robustness of CTE ID normalization, the developer refactored code to ensure normalization scopes remained isolated, preventing incorrect remapping of CTE references in intricate query scenarios. Using Java and Scala, they implemented targeted changes and introduced regression tests to validate the new behavior and guard against future regressions. Their work enhanced the reliability of Spark SQL’s CTE processing, improved test coverage, and clarified documentation around CTE semantics, contributing to more predictable query results in complex cases.