This is pure speculation. It's possible that using SELECT FOR UPDATE also locks the rows in the parent tables referenced in the field list. I believe this happened in older versions of PostgreSQL.
Interesting. In the query, paiyroll_endpoint.op_id and paiyroll_endpoint.client_id ARE foreign keys to other tables.
But I don't see any reference to locking rows in parent tables in the docs around https://www.postgresql.org/docs/current/explicit-locking.html#LOCKING-ROWS. A quick poke around did not reveal any documentation that confirms this one way or another. And to my admittedly in-expert thinking, it seems surprising that the parent might need to be locked?
On Saturday, March 7, 2026 at 04:25:01 AM GMT-5, Shaheed Haque <shaheedhaque@gmail.com> wrote:
Hi, I'm trying to understand/fix a rare deadlock in my application. Given my limited knowledge, what seems odd to me is that the deadlock involves two processes running exactly the same code/query, each of which (tries to) avoid issues by locking exactly one row for update. In Django-speak, the code does this:
# # Select-for-update exactly one row by id. # qs = Endpoint.objects.select_for_update().filter(id=instance.id) # # The above returns a queryset of one row which we loop over: # for item in qs:
...do stuff with item...
item.save() The deadlock is reported in the Postgres server log like this: ERROR: deadlock detected
DETAIL: Process 15576 waits for ShareLock on transaction 31053599; blocked by process 16953.
Process 16953 waits for ShareLock on transaction 31053597; blocked by process 15576.
Process 15576: SELECT “paiyroll_endpoint”.“id”, “paiyroll_endpoint”.“op_id”, “paiyroll_endpoint”.“client_id”, “paiyroll_endpoint”.“client_private”, “paiyroll_endpoint”.“netloc”, “paiyroll_endpoint”.“calls”, “paiyroll_endpoint”.“ms”, “paiyroll_endpoint”.“history”, “paiyroll_endpoint”.“current_history” FROM “paiyroll_endpoint” WHERE “paiyroll_endpoint”.“id” = 1 FOR UPDATE
Process 16953: SELECT “paiyroll_endpoint”.“id”, “paiyroll_endpoint”.“op_id”, “paiyroll_endpoint”.“client_id”, “paiyroll_endpoint”.“client_private”, “paiyroll_endpoint”.“netloc”, “paiyroll_endpoint”.“calls”, “paiyroll_endpoint”.“ms”, “paiyroll_endpoint”.“history”, “paiyroll_endpoint”.“current_history” FROM “paiyroll_endpoint” WHERE “paiyroll_endpoint”.“id” = 2 FOR UPDATE
HINT: See server log for query details.
CONTEXT: while locking tuple (7,15) in relation “paiyroll_endpoint”
STATEMENT: SELECT “paiyroll_endpoint”.“id”, “paiyroll_endpoint”.“op_id”, “paiyroll_endpoint”.“client_id”, “paiyroll_endpoint”.“client_private”, “paiyroll_endpoint”.“netloc”, “paiyroll_endpoint”.“calls”, “paiyroll_endpoint”.“ms”, “paiyroll_endpoint”.“history”, “paiyroll_endpoint”.“current_history” FROM “paiyroll_endpoint” WHERE “paiyroll_endpoint”.“id” = 1 FOR UPDATE How can there be a deadlock between updates to different rows (as per the bolded WHERE clauses)? Have I somehow turned off row-level locks? Is there some additional logging I could enable to try to catch the data needed to root-cause this?
