PostgreSQL – SELECT FOR UPDATE Locking Other Tables

lockingpostgresqlpostgresql-9.6

The situation, with PostgreSQL 9.6:

  • table A with integer primary key
  • table B with foreign key constraint
    on its primary key referencing table A's primary key

SELECT id FROM A FOR UPDATE; blocks UPDATE B SET x=y; until the lock on A is released.

I assume this is because the referenced value might change. What can I to avoid the first statement blocking execution of the second one without dropping the foreign key constraint?

If I drop the foreign key constraint, what bad things should I expect? In the actual database where this problem occurs, it wouldn't be a significant issue if table B had rows left over after deleting from A. We also don't ever update primary keys.

Edit with code:

-- Setup
create table a (id int primary key);
create table b (id int primary key references a (id) on update cascade on delete cascade, x varchar);
insert into a values (1), (2);
insert into b values (1, null), (2, null);

-- Left
begin;
select 1 from a for update;
/* Keep the transaction open */

-- Right
begin;
update b set x = 'abc' where id = 1;
update b set x = 'xyz'; /* It blocks here /*

Best Answer

We also don't ever update primary keys.

In that case, I think you can use FOR NO KEY UPDATE instead of FOR UPDATE. It's a weaker lock, as explained in Postgres docs about Explicit Locking:

FOR NO KEY UPDATE

Behaves similarly to FOR UPDATE, except that the lock acquired is weaker: this lock will not block SELECT FOR KEY SHARE commands that attempt to acquire a lock on the same rows. This lock mode is also acquired by any UPDATE that does not acquire a FOR UPDATE lock.

Test:

-- Setup
create table a (id int primary key, 
                x varchar);
create table b (id int primary key 
                    references a (id) on update cascade on delete cascade, 
                x varchar);
insert into a values (1), (2);
insert into b values (1, null), (2, null);

-- Left
begin;
select 1 from a for no key update;
/* Keep the transaction open */

            -- Right
            begin;
            update b set x = 'abc' where id = 1;
            update b set x = 'xyz';    -- doesn't block

-- Left
update a set x = 'left' where id = 1;
commit ;

            -- Right
            commit ;
Related Question