EXPLAIN Your SQL Query Plan

QuestDB is an open-source time-series database designed for high-performance workloads, offering ultra-low latency and high ingestion throughput with a multi-tier storage engine. It supports Parquet and SQL, ensuring data remains portable and AI-ready without vendor lock-in. The article introduces a series on optimizing SQL queries using the EXPLAIN keyword, which helps users understand and improve query performance by revealing the execution plan. The story recounts the author's journey in resolving performance issues in a Java application reliant on SQL queries, highlighting the challenges faced and the eventual discovery of the EXPLAIN command as a valuable tool. QuestDB processes queries through parsing, optimization, planning, and execution phases, and EXPLAIN provides insight into these processes, showcasing how different query structures and optimizations can significantly impact performance. Examples illustrate how variations in query construction, such as using Just-In-Time compilation or index scanning, can affect response times, emphasizing the importance of understanding database fundamentals and the execution plan for enhancing performance. Despite its utility, EXPLAIN does not indicate why a specific plan is chosen, but it remains an essential tool for query performance experimentation and validation.