Re: Combining Aggregates

On 17 December 2014 at 10:02, Atri Sharma <atri.jiit@gmail.com> wrote:
>
>
> On Wed, Dec 17, 2014 at 3:23 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>
>> KaiGai, David Rowley and myself have all made mention of various ways
>> we could optimize aggregates.
>>
>> Following WIP patch adds an extra function called a "combining
>> function", that is intended to allow the user to specify a
>> semantically correct way of breaking down an aggregate into multiple
>> steps.
>>
>> Gents, is this what you were thinking? If not...
>>
>>
>
> A quick look at the patch makes me assume that the patch does not handle the
> problem of combining transvals or move at all in that direction (which is
> fine, just reconfirming).

There are no applications of the new function at present. Each would
be similar, but unsure as to whether they would be identical.

> So, essentially, we are adding a "grand total" on top of individual sum() or
> count() operations,right?

The idea is to be able to do aggregation in multiple parts. For
example, allowing parallel query to have a plan like this

Aggregate
->Gather (subPlan is repeated N times on each parallel worker)  ->Aggregate     -->ParallelSeqScan (scans a distinct
portionof a table)
 

or to allow a serial plan where an aggregate was pushed down below a
join, like this

CURRENT PLAN
Aggregate
-> Join    -> Scan BaseTable1    -> Scan BaseTable2

PRE-AGGREGATED PLAN
Aggregate
-> Join    -> PreAggregate (doesn't call finalfn)         -> Scan BaseTable1    -> Scan BaseTable2

and also allow the above plan to be replaced by a Matview plan like this

Aggregate
-> Join    -> Scan BaseTable1.matviewA    -> Scan BaseTable2

where we would assume that the contents of matviewA are
pre-aggregations that could be further combined.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services