On 27 Srpen 2014, 21:41, Merlin Moncure wrote:
> On Wed, Aug 27, 2014 at 2:07 PM, Pavel Stehule <pavel.stehule@gmail.com>
>>
>> Are there some plans to use partitioning for aggregation?
>
> Besides min/max, what other aggregates (mean/stddev come to mind)
> would you optimize and how would you determine which ones could be?
> Where is that decision made?
>
> For example, could user defined aggregates be pushed down if you had a
> reaggregation routine broken out from the main one?
I think that what Pavel suggests is that when you are aggregating by
GROUP BY x
and 'x' happens to be used for partitioning (making it impossible to
groups from different partitions to overlap), then it's perfectly fine to
perform the aggregation per partition, and just append the results.
If you need sorted output, you can sort the results (assuming the
cardinality of the output is much lower than the actual data).
This "append first, then aggregate" may be the cause for switch to sort
(because of fear that the amount of group will exceed work_mem), while we
could just as fine process each partition by hash aggregate separately.
Tomas