On Wed, Apr 12, 2017 at 12:40 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Amit Kapila <amit.kapila16@gmail.com> writes:
>
>> I think there is a possibility of doing ExecInitNode in a parallel
>> worker for a parallel-unsafe subplan, because we pass a list of all
>> the sublans stored in planned statement.
>
> It's more than a possibility. If you run Andreas' test case against
> HEAD, now that the can't-transmit-RestrictInfo failure is fixed,
> you will find that the parallel worker actually creates and immediately
> destroys an FDW remote connection. (The easiest way to prove that is
> to turn on log_connections/log_disconnections. BTW, is there any
> convenient way to attach a debugger to a parallel worker process as it's
> being launched?
>
What I generally do to debug parallel worker case is to add a "while
(1) {}" kind of loop in the beginning of ParallelQueryMain() or in
ParallelWorkerMain() depending on area I want to debug, like in this
case it would be okay to add such a loop in ParallelQueryMain(). Then
as the worker will be waiting there attach the debugger to that
process and resume debugging. It is easier to identify the worker
process if we add an infinite loop, otherwise one can use sleep or
some other form of wait or debug break mechanism as well.
> I couldn't manage to catch the worker in the act.)
>
> So I think this is clearly a Bad Thing and we'd better do something
> about it. The consequences for postgres_fdw aren't so awful perhaps,
> but other non-parallel-safe FDWs might have bigger problems with this
> behavior.
>
>> However, the worker will
>> never execute such a plan as we don't generate a plan where unsafe
>> sublan/initplan is referenced in the node passed to the worker. If we
>> want to avoid passing parallel-unsafe subplans to workers, then I
>> think we can maintain a list of parallel safe subplans along with
>> subplans in PlannerGlobal and PlannedStmt or maybe keep a parallel
>> safe flag in Plan so that we can pass only parallel safe plans to
>> workers.
>
> Right, we could, say, leave a hole in the subplan list corresponding
> to any subplan that's not parallel-safe. That seems like a good idea
> anyway because right now there's clearly no cross-check preventing
> a worker from trying to run such a subplan.
>
> Anyone want to draft a patch for this?
>
Yes, I will work on it in this week and possibly today or tomorrow and
either produce a patch or if I face any problems, then will update
about them here.
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com