pg_stat_statements: everything it can't

Part one made the core case: pg_stat_statements counts, it doesn't record. It walked through how the queryid jumble fragments one logical query into many rows, how the first-seen text freezes your per-request tags, and how the averages bury the p99 that actually pages you. All of that was about data the extension has and distorts. This part is about the rest: the entries it silently throws away, the query text that can vanish all at once, the plans and replicas it never records, and the knobs that bite. It ends where part one started, with the question the whole investigation was really about: is this the query store Postgres is missing, or just the floor you'd build one on? The table fills up and evicts your tail pg_stat_statements.max defaults to 5000. It's a hard cap on entries, set when the server starts (changing it needs a restart, because the hash table is sized in shared memory up front). When the 5001st distinct shape arrives, Postgres doesn't grow the table. It evicts,…