Subject: Postgres attach partition: AccessExclusive lock set on different tables depending on how attaching is performed

From: user <[email-masked]>
Date: Sun, 20 Oct 2024 13:31:23 +0200

Hello,
I was reading all the tips that could make the attach partition operation
seamless. https://www.postgresql.org/docs/current/ddl-partitioning.html
There is a mention about check constraint that could be places before the
attach process. But to minimise the time when AccessExclusive lock is held
on my table, I wanted to push it further and also add indexes and foreign
keys BEFORE the attach command is invoked.
And here is a problem. When I run the attach command without foreign keys
being present beforehand on a table, there is only AccessExclusive lock on
a table I attach partition to.
BUT if my table to-be-attached has a foreign key constraint already, then
the referenced table will get the ExclusiveLock! I do not understand why is
it needed, the constraint already exists...

The reproduction: ( Postgres Version 14 )

CREATE TABLE refs (
                    id integer primary key,
                    did integer
                );
CREATE TABLE films (
                    id integer,
                    code char(5) ,
                    title varchar(40) NOT NULL,
                    did integer NOT NULL references refs(id)
                    )
                    partition by list (code);

insert into refs values (5, 5)
create table films_partition (LIKE films INCLUDING ALL)
case 1: films_partition does not have a foreign key added before the attach
BEGIN;
insert into films_partition values (1, 'dr', 'musician',5)
alter table films_partition add constraint check_code check (code = 'dr');
ALTER TABLE films ATTACH PARTITION films_partition for values in ('dr')
keep the transaction running...

check the locks:

select relname, mode
                from pg_locks l
                    join pg_class c on (relation = c.oid)
                    join pg_namespace nsp on (c.relnamespace = nsp.oid);
films relname, ShareUpdateExclusiveLock mode
films_partition relname, AccessShareLock mode
films_partition relname, RowExclusiveLock mode
films_partition relname, ShareRowExclusiveLock mode
films_partition relname, AccessExclusiveLock mode
refs relname, AccessShareLock mode
refs relname, RowShareLock mode
refs relname, ShareRowExclusiveLock mode

No AccessExclusive lock on "refs" table!

case 2: films_partition does have the foreign key contrain
 BEGIN;
insert into films_partition values (1, 'dr', 'musician',5)
alter table films_partition add constraint check_code check (code = 'dr');
alter table films_partition ADD CONSTRAINT fk_did FOREIGN KEY (did)
REFERENCES refs (id);
ALTER TABLE films ATTACH PARTITION films_partition for values in ('dr')
keep the transaction running...

check the locks:

films relname, ShareUpdateExclusiveLock mode
films_partition relname, AccessShareLock mode
films_partition relname, RowExclusiveLock mode
films_partition relname, ShareRowExclusiveLock mode
films_partition relname, AccessExclusiveLock mode
refs relname, AccessShareLock mode
refs relname, RowShareLock mode
refs relname, ShareRowExclusiveLock mode
refs relname, AccessExclusiveLock mode

There is AccessExclusiveLock on "refs" table!

Conclusion
I really don't want the "attach partition" to take too much time, so I want
to have all the constraints added before it is run. And indeed, the time is
reduced. But this additional lock now increases the chance of deadlocks, as
AccessExclusive locks are grabbed on many tables referenced by foreing
keys. Is there anything I can do better? Whi is it that attach_partition
adds a foreign key without additional AccessExclusive lock, but this lock
is required when the constrint already exists?

Regards!

---

From: Adrian Klaver <[email-masked]>
Date: Sun, 20 Oct 2024 09:23:19 -0700

On 10/20/24 04:31, user wrote:
> Hello,
> I was reading all the tips that could make the attach partition 
> operation seamless. 
> https://www.postgresql.org/docs/current/ddl-partitioning.html 
> <https://www.postgresql.org/docs/current/ddl-partitioning.html> There is 
> a mention about check constraint that could be places before the attach 
> process. But to minimise the time when AccessExclusive lock is held on 
> my table, I wanted to push it further and also add indexes and foreign 
> And here is a problem. When I run the attach command without foreign 
> keys being present beforehand on a table, there is only AccessExclusive 
> lock on a table I attach partition to.
> BUT if my table to-be-attached has a foreign key constraint already, 
> then the referenced table will get the ExclusiveLock! I do not 
> understand why is it needed, the constraint already exists...
> 
> I really don't want the "attach partition" to take too much time, so I 
> want to have all the constraints added before it is run. And indeed, the 
> time is reduced. But this additional lock now increases the chance of 
> deadlocks, as AccessExclusive locks are grabbed on many tables 
> referenced by foreing keys. Is there anything I can do better? Whi is it 
> that attach_partition adds a foreign key without additional 
> AccessExclusive lock,

https://www.postgresql.org/docs/current/sql-altertable.html

ATTACH PARTITION

[...]

"Currently FOREIGN KEY constraints are not considered. "

  but this lock is required when the constrint
> already exists?

Because I am pretty sure it is due to this statement:

Try:

BEGIN;
COMMIT;

Then:

Also from

https://www.postgresql.org/docs/current/sql-altertable.html

Take a look at:

ADD table_constraint [ NOT VALID ]

...

"Normally, this form will cause a scan of the table to verify that all 
existing rows in the table satisfy the new constraint. But if the NOT 
VALID option is used, this potentially-lengthy scan is skipped. The 
constraint will still be enforced against subsequent inserts or updates 
(that is, they'll fail unless there is a matching row in the referenced 
table, in the case of foreign keys, or they'll fail unless the new row 
matches the specified check condition). But the database will not assume 
that the constraint holds for all rows in the table, until it is 
validated by using the VALIDATE CONSTRAINT option. See Notes below for 
more information about using the NOT VALID option."

-- 
Adrian Klaver
[email-masked]