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
