Write PostgreSQL statements to: ( 1) update a flight's price and set updated_at correctly; ( 2) upsert a booking record with conflict handling; ( 3) select bookings filtered by a UTC date range using timestamptz without off-by-one errors; ( 4) add appropriate indexes and demonstrate their use with EXPLAIN ANALYZE. Discuss common pitfalls when moving from generic SQL to PostgreSQL, including date/time casting, time zones, text vs citext, and use of immutable/stable functions in indexes.