CedarDB, a new Postgres-compatible database developed from research at the Technical University of Munich, showcases impressive capabilities in query decorrelation. The author shares insights from testing CedarDB's handling of complex SQL queries, noting both strengths in its query planner and some early-stage issues. Overall, there is optimism about CedarDB's future as it continues to evolve.

SQL query optimization involves the DBMS determining the most efficient plan to execute a query, with the query optimizer responsible for evaluating different execution plans based on cost. The Plan Explorer tool, implemented for PostgreSQL, visualizes these plans and provides insights into the optimizer's decisions by generating various diagrams. The tool can operate in both standalone and server modes, enabling deeper analysis of query execution and costs.