Re: How to investigate deadlocks

On Mon, 2023-10-02 at 13:27 +0200, Matthias Apitz wrote:
> One of our clients running our LMS on top of PostgreSQL 13.1 created a
> ticket with these messages:
>
> 2023-09-30 16:50:50.951 CEST [18117] ERROR:  deadlock detected
> 2023-09-30 16:50:50.951 CEST [18117] DETAIL:  Process 18117 waits for ShareLock on transaction 150396154; blocked by process 18187.
>         Process 18187 waits for ShareLock on transaction 150396155; blocked by process 18117.
>         Process 18117: fetch hc_d03geb
>         Process 18187: fetch hc_d02ben
> 2023-09-30 16:50:50.951 CEST [18117] HINT:  See server log for query details.
> 2023-09-30 16:50:50.951 CEST [18117] CONTEXT:  while locking tuple (38,57) in relation "d03geb"
> 2023-09-30 16:50:50.951 CEST [18117] STATEMENT:  fetch hc_d03geb
>
> The shown PIDs for sure are the ones of the Pos backend proc (on Linux).
> Is there any chance to investigate it further?

See the line:

HINT: See server log for query details.

Other than that, it is difficult to figure out the exact cause of a deadlock.
You need to know all the statements that were run in these transactions,
which probably requires deeper knowledge of the application.

Yours,
Laurenz Albe