I attached one example of a partitioned table with multi column partition key. I also attached the output. Disabling the hash_join is not really necessary, it just shows the more drastic result in the case of low work_mem.
Comparing the first and the second query I was surprised to see that SET enable_partitionwise_join could cause the costs to go up. Shouldn't the paths of the first query be generated as well?
The third query seems to have a different issue. That one is close to my original performance problem. It looks to me like the push down of the sl condition stops the optimizer considering a partial join. If so would it be sane to keep a copy of the original quals to make the partial join possible? Do you have better ideas?