Arnaud Jegou focused on improving query planning in the apache/calcite repository by addressing a bug in the RelBuilder component. He fixed an issue where LIMIT clauses were incorrectly pushed through projections containing window functions, which previously led to invalid query plans. Using his expertise in Java development and database optimization, Arnaud updated the RelBuilder logic to ensure window function calculations are preserved when LIMIT is applied. He also implemented a regression test to validate the new behavior and prevent future regressions. This work demonstrated a deep understanding of query planning and careful attention to correctness in complex database optimization scenarios.