Locks in creating a partition in CREATE TABLE vs ALTER TABLE

[Asaf Flescher <asaf@xxxxxxxxx>]

Hi,

I'm not sure if this is a bug or I'm missing something regarding how partitioning is supposed to work but I've noticed (in Postgres 12.6) that if I have a partitioned table, and then try to add a partition to it via CREATE TABLE ... PARTITION OF, the statement will grab an AccessExclusive lock on the partitioned table. Meanwhile, if I create that same table normally, then attach it to the partitioned table via ALTER table - no AccessExclusive lock. Short recreation - 

In one session: 

CREATE TABLE stuff (stuff_id int) PARTITION BY LIST (stuff_id);

BEGIN;

SELECT * FROM stuff;

Then in a second session:

CREATE TABLE stuff_1 PARTITION OF stuff FOR VALUES IN (1); (Will get stuck, and a query on pg_locks will show an ungranted AccessExclusive lock).

CREATE TABLE stuff_1 (LIKE stuff); (Will work)

ALTER TABLE stuff ATTACH PARTITION stuff_1 FOR VALUES IN (1); (Will work)

Logically, the two approaches are doing the same thing, are they not? Or am I missing something?

Would appreciate any advice here,

Asaf