How do you optimize slow running SQL queries?

To optimize slow SQL queries, first identify the bottleneck by examining the execution plan with tools like EXPLAIN or EXPLAIN ANALYZE. Look for full table scans, missing indexes, or expensive joins. Add appropriate indexes on columns used in WHERE, JOIN, ORDER BY, and GROUP BY clauses, but avoid over‑indexing as it slows writes. Rewrite subqueries as joins or use EXISTS/IN when appropriate. Replace correlated subqueries with JOINs or window functions. Use SELECT only the columns needed instead of SELECT * to reduce I/O. Consider query refactoring: split a large query into smaller, materialized views or temporary tables if it processes huge data sets. For aggregation, ensure indexes support the grouping columns. If the database supports it, enable query caching or use materialized views. Finally, monitor the query after changes to confirm performance gains and adjust as data volume grows.