Constant AUTO_HASH_MIN_PAIRS 

pub const AUTO_HASH_MIN_PAIRS: usize = 64; // 64usize

Below this many candidate pairs, a nested loop is simply cheaper — building a hash table costs an allocation per distinct key and a clone of every right row’s key values, which a handful of comparisons does not repay.

Catalogue queries (psql’s \dt and friends) join a few dozen rows and stay on the reference path, which is a welcome side effect: the code path that real clients exercise most is the one whose semantics are most thoroughly tested.