Re: Combining Aggregates

On Thu, Jan 21, 2016 at 12:32 PM, David Rowley
<david(dot)rowley(at)2ndquadrant(dot)com> wrote:
> On 21 January 2016 at 04:59, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>
>> On Wed, Jan 20, 2016 at 7:53 AM, David Rowley
>> <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> > On 21 January 2016 at 01:44, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> >>
>> >> On Wed, Jan 20, 2016 at 7:38 AM, David Rowley
>> >> <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> >> >> To my mind, priority #1 ought to be putting this fine new
>> >> >> functionality to some use. Expanding it to every aggregate we've
>> >> >> got
>> >> >> seems like a distinctly second priority. That's not to say that
>> >> >> it's
>> >> >> absolutely gotta go down that way, but those would be my priorities.
>> >> >
>> >> > Agreed. So I've attached a version of the patch which does not have
>> >> > any
>> >> > of
>> >> > the serialise/deserialise stuff in it.
>> >> >
>> >> > I've also attached a test patch which modifies the grouping planner
>> >> > to
>> >> > add a
>> >> > Partial Aggregate node, and a final aggregate node when it's
>> >> > possible.
>> >> > Running the regression tests with this patch only shows up variances
>> >> > in
>> >> > the
>> >> > EXPLAIN outputs, which is of course expected.
>> >>
>> >> That seems great as a test, but what's the first patch that can put
>> >> this to real and permanent use?
>> >
>> > There's no reason why parallel aggregates can't use the
>> > combine_aggregate_state_d6d480b_2016-01-21.patch patch.
>>
>> I agree. Are you going to work on that? Are you expecting me to work
>> on that? Do you think we can use Haribabu's patch? What other
>> applications are in play in the near term, if any?
>
>
> At the moment I think everything which will use this is queued up behind the
> pathification of the grouping planner which Tom is working on. I think
> naturally Parallel Aggregate makes sense to work on first, given all the
> other parallel stuff in this release. I plan on working on that that by
> either assisting Haribabu, or... whatever else it takes.
>

Here I attached updated patch of parallel aggregate based on the latest
changes in master. Still it lack of cost comparison of normal aggregate to
parallel aggregate because of difficulty. This cost comparison is required
in parallel aggregate as this is having some regression when the number
of groups are less in the query plan.

Regards,
Hari Babu
Fujitsu Australia