Tom Lane schrieb am 06.06.2018 um 16:32:
> Thomas Kellerer <spam_eater@gmx.net> writes:
>> Is this a known limitation?
>
> Yes, unless somebody has done radical restructuring of the aggregation
> code while I wasn't looking.
>
> agg(DISTINCT ...) is currently implemented inside the Agg plan node,
> so it's an indivisible black box to everything else. That was a
> simple, minimum-code-footprint method for implementing the feature
> back when; but it's got lots of drawbacks, and one is that there's
> no reasonable way to parallelize.
>
> I'd anticipate that before we could even start to think of parallelizing,
> we'd have to split out the distinct-ification processing into a separate
> plan node.
>
> agg(... ORDER BY ...) has got the same problem, and it'd likely be
> advisable to fix that at the same time.
Thansk for the explanation.