This is mostly a cosmetic change. When only 1 child is present there
is no need for ordered append. In this situation we might still
benefit from a ChunkAppend node here due to runtime chunk exclusion
when we have non-immutable constraints, so we still add the ChunkAppend
node in that situation even with only 1 child.
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
On PG15 CustomScan by default is not projection capable, thus wraps this
node in Result node. THis change in PG15 causes tests result files which
have EXPLAIN output to fail. This patch fixes the plan outputs.
Fixes#4833
