I have forgotten to mention that I have enable_partitionwise_aggregate=on
in the global settings since the beginning. According to the docs:
> Enables or disables the query planner's use of partitionwise grouping or
> aggregation, which allows grouping or aggregation on partitioned tables
> to be performed separately for each partition.
Reading that, I'd expect to see a separate DISTINCT->LIMIT 10 on every
partition, and then it would be up to independent plans to decide whether
each partition follows a parallel or a serial plan.
Not sure if this plan was checked but rejected because of cost.
Dimitris