Postgres EXPLAIN, in ten lines

EXPLAIN (ANALYZE, BUFFERS, VERBOSE, FORMAT TEXT) <query>;

• Seq Scan: full table read. Fine on small tables.
• Index Scan: uses an index; still fetches rows.
• Index Only Scan: served from the index alone.
• Bitmap Heap Scan: combines multiple indexes.
• Rows Removed by Filter: index didn't cover the predicate.
• Buffers: shared read: cold cache; consider warm-up.