INSERT .. SELECT query containing distributed hypertables generates plan
with DataNodeCopy node which is not supported. Issue is in function
tsl_create_distributed_insert_path() where we decide if we should
generate DataNodeCopy or DataNodeDispatch node based on the kind of
query. In PG15 for INSERT .. SELECT query timescaledb planner generates
DataNodeCopy as rte->subquery is set to NULL. This is because of a commit
in PG15 where rte->subquery is set to NULL as part of a fix.
This patch checks if SELECT subquery has distributed hypertables or not
by looking into root->parse->jointree which represents subquery.
Fixes#4983
bfa641a81c