Database indexes on foreign keys are not automatic in PostgreSQL

Unlike MySQL, PostgreSQL does NOT automatically create indexes on foreign key columns. This means your JOIN queries on user_id, post_id, category_id, etc. are doing sequential scans. Run EXPLAIN ANALYZE on your slow queries — if you see Seq Scan on a table with >10K rows, add a B-tree index. This single oversight causes more production performance issues than any other PostgreSQL mistake.