Re: Unexpected deadlock across two separate rows, using Postgres 17 and Django's select_for_update()

On Mon, Mar 9, 2026 at 5:55 AM Shaheed Haque <shaheedhaque(at)gmail(dot)com> wrote:

> I'm not sure I understand. The two queries are referencing separate,
> single rows in the child table (primary keys payroll_endpoint.id = 1 and
> 2), so where does the multi-row bit come in? Is it because the two parent
> tables are also being locked, in possibly different orders?
>

This is unrelated to parent tables. What is not shown is the previous
locks. Process A locks id 1. Process B locks id 2, then attempts to lock id
1. Process A attempts to lock id 2. Hence, a deadlock, even if all four
lock attempts are not shown in the log.

I'm not seeing "Tom's suggestion". Is there a way to specify that the
> parent tables need not be locked? Perhaps by omitting them from the query?
>

alter system set log_statement = 'all';
select pg_reload_conf();

## Run your program and get the deadlock error

alter system reset log_statement;
select pg_reload_conf();

Now check your logs, find the PIDs involved in the deadlock, and trace what
actions they did before the deadlock occurred.

Here's a real example, showing what the Postgres logs will look like:

-- Process A:
create table t (id int);
insert into t values (1),(2);
begin;
select * from t where id = 1 for update;

-- Process B:
begin;
select * from t where id = 2 for update;
select * from t where id = 1 for update; -- Hangs, waiting for process A to
finish

-- Process A:
select * from t where id = 2 for update;

DEADLOCK!

Logs:

2026-03-09 09:00:23.885 EDT [242171] LOG: statement: alter system set
log_statement = 'all';
2026-03-09 09:00:25.804 EDT [242136] LOG: received SIGHUP, reloading
configuration files
2026-03-09 09:00:25.805 EDT [242136] LOG: parameter "log_statement"
changed to "all"
2026-03-09 09:00:31.910 EDT [242171] LOG: statement: drop table if exists
t;
2026-03-09 09:00:36.649 EDT [242171] LOG: statement: create table t(id
int);
2026-03-09 09:00:39.522 EDT [242171] LOG: statement: insert into t values
(1),(2);
2026-03-09 09:00:42.121 EDT [242171] LOG: statement: begin;
2026-03-09 09:00:50.788 EDT [242171] LOG: statement: select * from t where
id=1 for update;
2026-03-09 09:00:59.755 EDT [242176] LOG: statement: begin;
2026-03-09 09:01:05.509 EDT [242176] LOG: statement: select * from t where
id=2 for update;
2026-03-09 09:01:08.397 EDT [242176] LOG: statement: select * from t where
id=1 for update;
2026-03-09 09:01:14.278 EDT [242171] LOG: statement: select * from t where
id=2 for update;
2026-03-09 09:01:15.279 EDT [242171] ERROR: deadlock detected
2026-03-09 09:01:15.279 EDT [242171] DETAIL: Process 242171 waits for
ShareLock on transaction 15122348; blocked by process 242176.
Process 242176 waits for ShareLock on transaction 15122347; blocked
by process 242171.
Process 242171: select * from t where id=2 for update;
Process 242176: select * from t where id=1 for update;
2026-03-09 09:01:15.279 EDT [242171] HINT: See server log for query
details.
2026-03-09 09:01:15.279 EDT [242171] CONTEXT: while locking tuple (0,2) in
relation "t"
2026-03-09 09:01:15.279 EDT [242171] STATEMENT: select * from t where id=2
for update;

Cheers,
Greg

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