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

Hello,
Sorry for nagging, but I would really like to find some answers.
So, to reiterate. Experiment done as follows:
"""""""""""""""""""""'
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)
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);
"""""""""""""""""""""""""
Then, when we open a transaction and try to attach:
"""""""""""""""""""""""""
BEGIN;
ALTER TABLE films ATTACH PARTITION films_partition for values in ('dr')
keep the transaction running..
""""""""""""""""""""""""
Once we check a locks, we will see that there is AccessExclusiveLock on
table refs.
""""""
select relname, mode
from pg_locks l
join pg_class c on (relation = c.oid)
join pg_namespace nsp on (c.relnamespace = nsp.oid);
"""""
My questions are:
1. Why is postgres adding again a constraint? Can't it detect that foreign
key already exists? I want to avoid locking partitioned table for too long.
2. Even when attach is adding a foreign key again, why is there
AccessExclusiveLock on refs table? foreign key constraint addition does not
require it.
https://pglocks.org/?pgcommand=ALTER%20TABLE%20ADD%20FOREIGN%20KEY%20(CHILD)
3. If I repeat the steps listed above, but do not add foreign key manually,
then attach partition does not hold AccessExclusive lock on table refs. It
still needs to add a foreign key, as "films" table has that constraint. Why
is the AccessExclusive lock missing from "refs" table now?

Regards!

---------- Forwarded message ---------
From: user <user(at)pidu(dot)dev>
Date: Fri, 1 Nov 2024 at 15:35
Subject: Re: Fwd: Postgres attach partition: AccessExclusive lock set on
different tables depending on how attaching is performed
To: <adrian(dot)klaver(at)aklaver(dot)com>
Cc: <pgsql-general(at)lists(dot)postgresql(dot)org>

Hello Adrian,
My apology for answering so late.

Indeed I have not splitted my transactions correctly in my first example.
BUT, if you change the syntaxt so that attach is performed in its own
transaction, result is the same. Try it out!

Also you have told me that I am seeing ExclusiveLock on refs table because
it is from foreign key constraint. Well this is incorrect, foreign key does
not take the ExclusiveLock in any situation.
https://pglocks.org/?pgcommand=ALTER%20TABLE%20ADD%20FOREIGN%20KEY%20(CHILD)

Could you try again with only attach being in its own transaction? You
should reproduce it.
Regards

On Mon, 21 Oct 2024 at 20:31, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

>
>
> On 10/21/24 1:40 AM, user wrote:
> > ** forwarding to mailing list, forgot to add header
> >
> >
> > Thanks for answering.
> > I think one misunderstanding happened.
> > The parent table has the foreign key constraint.
> > So attach partition will add this constraint for table being attached.
> > (How this compares to foreign keys not being considered, not sure).
> >
> > Why is it that attach_partition does not require exclusive lock when
> > creating a constraint automatically?
> >
> > What is more, you have provided a quote that states the lock is needed
> > because the table needs to be checked that all entries comply with the
> > NEW constraint.
> >
> > Well it is not new when I manually create it before I attach.
> > It is new when I run attach command without previous manual constraint
> > creation, but then the lock is not created.
>
> 1) Case 1
>
>
> test=# \d films
> Partitioned table "public.films"
> Column | Type | Collation | Nullable | Default
> --------+-----------------------+-----------+----------+---------
> id | integer | | |
> code | character(5) | | |
> title | character varying(40) | | not null |
> did | integer | | not null |
> Partition key: LIST (code)
> Foreign-key constraints:
> "films_did_fkey" FOREIGN KEY (did) REFERENCES refs(id)
> Number of partitions: 0
>
> create table films_partition (LIKE films INCLUDING ALL);
> CREATE TABLE
>
> test=# \d+ films_partition
> Table "public.films_partition"
> Column | Type | Collation | Nullable | Default |
> Storage | Compression | Stats target | Description
>
> --------+-----------------------+-----------+----------+---------+----------+-------------+--------------+-------------
> id | integer | | | |
> plain | | |
> code | character(5) | | | |
> extended | | |
> title | character varying(40) | | not null | |
> extended | | |
> did | integer | | not null | |
> plain | | |
> Access method: heap
>
>
> ALTER TABLE films ATTACH PARTITION films_partition for values in ('dr');
> ALTER TABLE
>
> test=# \d+ films
> Partitioned table
> "public.films"
> Column | Type | Collation | Nullable | Default |
> Storage | Compression | Stats target | Description
>
> --------+-----------------------+-----------+----------+---------+----------+-------------+--------------+-------------
> id | integer | | | |
> plain | | |
> code | character(5) | | | |
> extended | | |
> title | character varying(40) | | not null | |
> extended | | |
> did | integer | | not null | |
> plain | | |
> Partition key: LIST (code)
> Foreign-key constraints:
> "films_did_fkey" FOREIGN KEY (did) REFERENCES refs(id)
> Partitions: films_partition FOR VALUES IN ('dr ')
>
> test=# \d+ films_partition
> Table "public.films_partition"
> Column | Type | Collation | Nullable | Default |
> Storage | Compression | Stats target | Description
>
> --------+-----------------------+-----------+----------+---------+----------+-------------+--------------+-------------
> id | integer | | | |
> plain | | |
> code | character(5) | | | |
> extended | | |
> title | character varying(40) | | not null | |
> extended | | |
> did | integer | | not null | |
> plain | | |
> Partition of: films FOR VALUES IN ('dr ')
> Partition constraint: ((code IS NOT NULL) AND (code = 'dr
> '::character(5)))
> Check constraints:
> "check_code" CHECK (code = 'dr'::bpchar)
> Foreign-key constraints:
> TABLE "films" CONSTRAINT "films_did_fkey" FOREIGN KEY (did)
> REFERENCES refs(id)
> Access method: heap
>
>
> 2) Case 2
>
> create table films_partition (LIKE films INCLUDING ALL);
>
> 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);
>
> test=# \d films_partition
> Table "public.films_partition"
> Column | Type | Collation | Nullable | Default
> --------+-----------------------+-----------+----------+---------
> id | integer | | |
> code | character(5) | | |
> title | character varying(40) | | not null |
> did | integer | | not null |
> Check constraints:
> "check_code" CHECK (code = 'dr'::bpchar)
> Foreign-key constraints:
> "fk_did" FOREIGN KEY (did) REFERENCES refs(id)
>
> ****Note the FK definition***
>
> test=# ALTER TABLE films ATTACH PARTITION films_partition for values in
> ('dr');
>
> test=# \d films_partition
> Table "public.films_partition"
> Column | Type | Collation | Nullable | Default
> --------+-----------------------+-----------+----------+---------
> id | integer | | |
> code | character(5) | | |
> title | character varying(40) | | not null |
> did | integer | | not null |
> Partition of: films FOR VALUES IN ('dr ')
> Check constraints:
> "check_code" CHECK (code = 'dr'::bpchar)
> Foreign-key constraints:
> TABLE "films" CONSTRAINT "films_did_fkey" FOREIGN KEY (did)
> REFERENCES refs(id)
>
> ****Note the FK definition***
>
>
> What you are seeing is the locking for
>
> alter table films_partition ADD CONSTRAINT fk_did FOREIGN KEY (did)
> REFERENCES refs (id);
>
> At this point films_partition is a stand alone table that you are
> creating a FK back to refs. The ALTER TABLE films_partition ADD
> CONSTRAINT command has no knowledge of the target table you are going to
> attach films_partition to. When you do the ATTACH then a new FK is
> created just the same as in Case 1.
>
> >
> >
> > On Sun, 20 Oct 2024, 18:23 Adrian Klaver, <adrian(dot)klaver(at)aklaver(dot)com
> > <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:
> >
> > 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>
> > > <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
> > > 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,
> >
> > https://www.postgresql.org/docs/current/sql-altertable.html
> > <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:
> >
> > alter table films_partition ADD CONSTRAINT fk_did FOREIGN KEY (did)
> > REFERENCES refs (id);
> >
> > Try:
> >
> > 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);
> > COMMIT;
> >
> > Then:
> >
> > ALTER TABLE films ATTACH PARTITION films_partition for values in
> ('dr')
> >
> >
> > Also from
> >
> > https://www.postgresql.org/docs/current/sql-altertable.html
> > <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."
> >
> >
> > >
> > > Regards!
> > >
> > >
> >
> > --
> > Adrian Klaver
> > adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>
> >
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>