Re: BUG #15290: Stuck Parallel Index Scan query

On Mon, Jul 23, 2018 at 7:57 PM, Victor Yegorov <vyegorov@gmail.com> wrote:
> - `ERROR:  canceling statement due to conflict with recovery`, happened
> right when our problematic query started, same user

Ok, so that would explain how the master was cancelled.  In 2877's
stack we see that it was aborting here:

#11 0x00007f539697ba5e in PostgresMain (argc=1,
argv=argv@entry=0x7f5398d1bbc8, dbname=0x7f5398d1bb98 "coub",
username=0x7f5398d1bbb0 "app") at
/build/postgresql-10-U6N320/postgresql-10-10.4/build/../src/backend/tcop/postgres.c:3879

That line calls AbortCurrentTransaction(), just after the call to
EmitErrorReport() that wrote something in your log.  Andres's theory
(interrupts 'held') seems promising... perhaps there could be a bug
where parallel index scans leak a share-locked page or something like
that.  I tried to reproduce this a bit, but no cigar so far.  I wonder
if there could be something about your bloated index that reaches
buggy behaviour...

If you happen to have a core file for a worker that is waiting in
ConditionVariableSleep(), or it happens again, you'd be able to see if
an LWLock is causing this by printing num_held_lwlocks.