The HOT chapter is the one with the most direct practical payoff. Two things worth knowing from it: HOT only kicks in when no indexed column changes and the new tuple fits on the same page, so an index on a frequently-updated column (status, updated_at) quietly disables it for every update to that table. And lowering fillfactor (say 80-90) on update-heavy tables leaves free space on each page so HOT can actually happen. Comparing n_tup_hot_upd to n_tup_upd in pg_stat_user_tables is a cheap way to see whether you're getting HOT updates before tuning anything.