attached copy_par_regress_test.sql is a simple benchmark sql file, a partitioned table with 10 partitions, 2 levels of indirection. The simple benchmark shows around 7.7% improvement in my local environment.
I confirm that the patch introduces some improvement in simple cases like the one you shared. I looked around a bit to understand whether there is an obvious reason why copying from a partitioned table is not allowed, but couldn't find one. It seems ok to me.
I realized that while both "COPY <partitioned_table> TO..." and "COPY (SELECT..) TO..." can return the same set of rows, their orders may not be the same. I guess that it's hard to guess in which order find_all_inheritors() would return tables, and that might be something we should be worried about with the patch. What do you think?
