Re: Partial aggregates pushdown

Peter Eisentraut писал 2021-11-01 12:47:
> On 21.10.21 12:55, Alexander Pyhalov wrote:
>> Now aggregates with internal states can be pushed down, if they are
>> marked as pushdown safe (this flag is set to true for min/max/sum),
>> have internal states and associated converters. Converters are called
>> locally, they transform aggregate result to serialized internal
>> representation.
>> As converters don't have access to internal aggregate state, partial
>> aggregates like avg() are still not pushable.
>
> It seems to me that the system should be able to determine from the
> existing aggregate catalog entry whether an aggregate can be pushed
> down. For example, it could check aggtranstype != internal and
> similar. A separate boolean flag should not be necessary.

Hi.
I think we can't infer this property from existing flags. For example,
if I have avg() with bigint[] argtranstype, it doesn't mean we can push
down it. We couldn't also decide if partial aggregete is safe to push
down based on aggfinalfn presence (for example, it is defined for
sum(numeric), but we can push it down.

> Or if it
> is, the patch should provide some guidance about how an aggregate
> function author should set it.

Where should it be provided?

--
Best regards,
Alexander Pyhalov,
Postgres Professional