What is EXPLAIN in PostgreSQL?

EXPLAIN displays the query execution plan that the PostgreSQL query planner will use for a query. EXPLAIN SELECT * FROM users WHERE email = 'a@b.com'; shows the plan without executing. EXPLAIN ANALYZE executes the query and shows actual timings alongside estimated ones — essential for diagnosing performance. Key plan nodes: Seq Scan (full table scan), Index Scan (uses index), Index Only Scan (data from index only — fastest), Bitmap Heap Scan (batch index+heap reads), Hash Join/Merge Join/Nested Loop (join algorithms). Look at cost, rows estimates vs actuals, and where estimates are far off (stale statistics). Run ANALYZE tablename to update statistics.