Use EXPLAIN to Understand Query Plans

The EXPLAIN command is one of the most important tools in your SQL optimization toolkit. It helps you visualize how the database executes your query—including which indexes are used, how joins are performed, and which steps are the most expensive. Without it, optimizing SQL performance is mostly guesswork.

Running EXPLAIN (or EXPLAIN ANALYZE in some databases) before a query gives you a breakdown of the execution plan, showing whether your query triggers a full table scan, uses an index, performs a nested loop join, or spills to disk. These details make it easier to debug slow queries and find improvement opportunities.

For example, if EXPLAIN reveals a sequential scan on a million-row table, you may need to add an index or rewrite the WHERE clause. Similarly, if you see an inefficient join method like a nested loop on large inputs, you can restructure the query or provide better join keys.

Galaxy visualizes execution plans with AI-powered annotations, so you can understand exactly what’s slowing things down. Whether you're debugging one query or tuning hundreds, using EXPLAIN is a foundational habit that can lead to 2x–10x performance improvements depending on what you discover.