Re: Parallel Seq Scan

On 1/11/15 3:57 PM, Robert Haas wrote:
> On Sun, Jan 11, 2015 at 5:27 AM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>> * Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
>>> On Thu, Jan 8, 2015 at 2:46 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>>>> Yeah, if we come up with a plan for X workers and end up not being able
>>>> to spawn that many then I could see that being worth a warning or notice
>>>> or something. Not sure what EXPLAIN has to do anything with it..
>>>
>>> That seems mighty odd to me. If there are 8 background worker
>>> processes available, and you allow each session to use at most 4, then
>>> when there are >2 sessions trying to do parallelism at the same time,
>>> they might not all get their workers. Emitting a notice for that
>>> seems like it would be awfully chatty.
>>
>> Yeah, agreed, it could get quite noisy. Did you have another thought
>> for how to address the concern raised? Specifically, that you might not
>> get as many workers as you thought you would?
>
> I'm not sure why that's a condition in need of special reporting.

The case raised before (that I think is valid) is: what if you have a query that is massively parallel. You expect it to get 60 cores on the server and take 10 minutes. Instead it gets 10 and takes an hour (or worse, 1 and takes 10 hours).

Maybe it's not worth dealing with that in the first version, but I expect it will come up very quickly. We better make sure we're not painting ourselves in a corner.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com