Re: [HACKERS] SERIALIZABLE with parallel query

On Fri, Feb 23, 2018 at 7:56 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Fri, Feb 23, 2018 at 8:48 AM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> On Fri, Feb 23, 2018 at 3:29 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> By the way, in which case leader can exit early? As of now, we do
>>> wait for workers to end both before the query is finished or in error
>>> cases.
>>
>> create table foo as select generate_series(1, 10)::int a;
>> alter table foo set (parallel_workers = 2);
>> set parallel_setup_cost = 0;
>> set parallel_tuple_cost = 0;
>> select count(a / 0) from foo;
>>
>> That reliably gives me:
>> ERROR: division by zero [from leader]
>> ERROR: could not map dynamic shared memory segment [from workers]
>>
>> I thought this was coming from resource manager cleanup, but you're
>> right: that happens after we wait for all workers to finish. Perhaps
>> this is a race within DestroyParallelContext() itself: when it is
>> called by AtEOXact_Parallel() during an abort, it asks the postmaster
>> to SIGTERM the workers, then it immediately detaches from the DSM
>> segment, and then it waits for the worker to start up.
>>
>
> I guess you mean to say worker waits to shutdown/exit. Why would it
> wait for startup at that stage?

Right, I meant to say shutdown/exit.

--
Thomas Munro
http://www.enterprisedb.com