Re: Parallel Seq Scan

On Fri, Jan 9, 2015 at 1:02 AM, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com> wrote:
>
> On 1/5/15, 9:21 AM, Stephen Frost wrote:
>>
>> * Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
>>>
>>> I think it's right to view this in the same way we view work_mem. We
>>> plan on the assumption that an amount of memory equal to work_mem will
>>> be available at execution time, without actually reserving it.
>>
>>
>> Agreed- this seems like a good approach for how to address this. We
>> should still be able to end up with plans which use less than the max
>> possible parallel workers though, as I pointed out somewhere up-thread.
>> This is also similar to work_mem- we certainly have plans which don't
>> expect to use all of work_mem and others that expect to use all of it
>> (per node, of course).
>
>
> I agree, but we should try and warn the user if they set
parallel_seqscan_degree close to max_worker_processes, or at least give
some indication of what's going on. This is something you could end up
beating your head on wondering why it's not working.
>

Yet another way to handle the case when enough workers are not
available is to let user specify the desired minimum percentage of
requested parallel workers with parameter like
PARALLEL_QUERY_MIN_PERCENT. For example, if you specify
50 for this parameter, then at least 50% of the parallel workers
requested for any parallel operation must be available in order for
the operation to succeed else it will give error. If the value is set to
null, then all parallel operations will proceed as long as at least two
parallel workers are available for processing.

This is something how other commercial database handles such a
situation.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com