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