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

Thank you for the replies.

On Sun, 8 Mar 2026 at 22:08, Greg Sabino Mullane <htamfids(at)gmail(dot)com> wrote:

> On Sun, Mar 8, 2026 at 2:23 PM <felix(dot)quintgz(at)yahoo(dot)com> wrote:
>
>> What makes me suspect it's a lock on the parent table is the word
>> "ShareLock" in the logs. A SELECT ... FOR UPDATE statement shouldn't place
>> that type of lock on the table it's selecting.
>>
>
> This looks 100% like a normal, multi-row deadlock situation. The CONTEXT
> shows it is a row-level problem:
>

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?

>
> CONTEXT: while locking tuple (7,15) in relation “paiyroll_endpoint”
>
>
> The ShareLocks are on the transaction, because each backend is waiting for
> the other to finish their transaction, and thus release the lock(s) it may
> have.
>
> If you implement Tom's suggestion, I think you will find that this is a
> classic failing to lock the rows in the same order problem.
>

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?

Thanks, Shaheed

>
> Cheers,
> Greg
>
>