Worked on enhancing the reliability of query planning in Percona Server by addressing a bug related to EXPLAIN handling for derived-table subqueries involving stored functions. Focused on ensuring that stored functions within derived tables were not incorrectly evaluated as constant expressions during EXPLAIN, which previously led to unintended function execution and inaccurate query plans. Applied and validated a targeted fix in the percona/percona-server repository, improving the accuracy of EXPLAIN outputs in complex subquery scenarios. Utilized skills in database optimization, SQL, and C++ to contribute to the stability and correctness of the query planner, emphasizing careful validation and precise implementation.