Can you imagine, at least theoretically, any kind of query on a
physical replica (hot standby) which would block other queries on the
same replica?
The only thing that comes to mine is advisory locks and also queries can block replication replay.
prod=# select pg_is_in_recovery();
-[ RECORD 1 ]-----+--
pg_is_in_recovery | t
prod=# select * from blocked_session;
-[ RECORD 1 ]---+-----------------------------
pid | 39330
tree | 39330
root | 39330
level | 0
user | rui
state | idle
state_duration | 00:01:01.663597
wait_event_type | Client
wait_event | ClientRead
lockmode | ExclusiveLock
locktype | advisory
granted | t
lock_wait |
blocked | 1
query | select pg_advisory_lock(42);
objects |
-[ RECORD 2 ]---+-----------------------------
pid | 39327
tree | 39330.39327
root | 39330
level | 1
user | rui
state | active
state_duration | 00:00:51.407894
wait_event_type | Lock
wait_event | advisory
lockmode | ExclusiveLock
locktype | advisory
granted | f
lock_wait | 00:00:51.407746
blocked | 0
query | select pg_advisory_lock(42);
objects |