Full table lock dropping a foreign key

Hello,

I noticed that Postgres takes an AccessExclusiveLock (a lock on the 
whole table) against the *referenced* table when dropping a foreign key. 
I wasn't expecting that, and some experimentation showed it does *not* 
take one when creating the FK. For example:

pjtest=# create table parent (id integer primary key);
CREATE TABLE
pjtest=# create table child (id integer primary key, parent_id integer);
CREATE TABLE
pjtest=# begin;
BEGIN
pjtest=# alter table child add constraint pfk foreign key (parent_id) 
references parent (id);
ALTER TABLE

And now pg_locks has this:

pjtest=# select locktype, relation::regclass, mode from pg_locks;
    locktype    |  relation   |         mode
---------------+-------------+-----------------------
  relation      | parent_pkey | AccessShareLock
  relation      | child_pkey  | AccessShareLock
  virtualxid    |             | ExclusiveLock
  relation      | pg_locks    | AccessShareLock
  virtualxid    |             | ExclusiveLock
  relation      | parent      | AccessShareLock
  relation      | parent      | RowShareLock
  relation      | parent      | ShareRowExclusiveLock
  transactionid |             | ExclusiveLock
  relation      | child       | AccessShareLock
  relation      | child       | ShareRowExclusiveLock
(11 rows)

But after dropping it:

pjtest=# commit;
COMMIT
pjtest=# begin;
BEGIN
pjtest=# alter table child drop constraint pfk;
ALTER TABLE

Now my locks are:

pjtest=# select locktype, relation::regclass, mode from pg_locks;
    locktype    | relation |        mode
---------------+----------+---------------------
  virtualxid    |          | ExclusiveLock
  relation      | pg_locks | AccessShareLock
  virtualxid    |          | ExclusiveLock
  relation      | parent   | AccessExclusiveLock
  relation      | child    | AccessExclusiveLock
  transactionid |          | ExclusiveLock
  object        |          | AccessExclusiveLock
  object        |          | AccessExclusiveLock
  object        |          | AccessExclusiveLock
  object        |          | AccessExclusiveLock
  object        |          | AccessExclusiveLock
(11 rows)

I noticed this on 9.5 but confirmed it on 10.5.

I was surprised because the docs give a pretty short list of things that 
take AccessExclusiveLocks 
(https://www.postgresql.org/docs/current/static/explicit-locking.html). 
It mentions ALTER TABLE, and it makes sense when I recall that foreign 
keys are implemented by putting triggers on *both* referencing & 
referenced tables, but still it caught me off guard. Also I don't 
understand why the lock is not necessary when adding a foreign key?

Anyway I don't have much of a question, although I wouldn't mind adding 
a note to the docs that dropping an FK takes this lock on both tables, 
if others agree that is a good idea.

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com