Re: Parallel Seq Scan

On 2015-04-21 AM 03:29, Robert Haas wrote:
> On Wed, Apr 8, 2015 at 3:38 AM, Amit Langote wrote:
>> On 08-04-2015 PM 12:46, Amit Kapila wrote:
>>> Going forward, I think we can improve the same if we decide not to shutdown
>>> parallel workers till postmaster shutdown once they are started and
>>> then just allocate them during executor-start phase.
>>
>> I wonder if it makes sense to invent the notion of a global pool of workers
>> with configurable number of workers that are created at postmaster start and
>> destroyed at shutdown and requested for use when a query uses parallelizable
>> nodes.
> 
> Short answer: Yes, but not for the first version of this feature.
> 
> Longer answer: We can't actually very reasonably have a "global" pool
> of workers so long as we retain the restriction that a backend
> connected to one database cannot subsequently disconnect from it and
> connect to some other database instead.  However, it's certainly a
> good idea to reuse the same workers for subsequent operations on the
> same database, especially if they are also by the same user.  At the
> very minimum, it would be good to reuse the same workers for
> subsequent operations within the same query, instead of destroying the
> old ones and creating new ones.  Notwithstanding the obvious value of
> all of these ideas, I don't think we should do any of them for the
> first version of this feature.  This is too big a thing to get perfect
> on the first try.
> 

Agreed.

Perhaps, Amit has worked (is working) on "reuse the same workers for
subsequent operations within the same query"

Thanks,
Amit