I noticed something else interesting. If you try an actually-useful
UPDATE, ie one that has to do some computation in the target list,
you can get a plan like this if it's a partitioned table:
EXPLAIN (verbose, costs off) UPDATE parent SET f2 = f2 + 1;
QUERY PLAN
---------------------------------------------------------------------------
Update on public.parent
Update on public.child1 parent_1
Update on public.child2 parent_2
Update on public.child3 parent_3
-> Append
-> Seq Scan on public.child1 parent_1
Output: (parent_1.f2 + 1), parent_1.tableoid, parent_1.ctid
-> Seq Scan on public.child2 parent_2
Output: (parent_2.f2 + 1), parent_2.tableoid, parent_2.ctid
-> Seq Scan on public.child3 parent_3
Output: (parent_3.f2 + 1), parent_3.tableoid, parent_3.ctid
But when using traditional inheritance, it looks more like:
EXPLAIN (verbose, costs off) UPDATE parent SET f2 = f2 + 1;
QUERY PLAN
---------------------------------------------------------------------------
Update on public.parent
Update on public.parent parent_1
Update on public.child1 parent_2
Update on public.child2 parent_3
Update on public.child3 parent_4
-> Result
Output: (parent.f2 + 1), parent.tableoid, parent.ctid
-> Append
-> Seq Scan on public.parent parent_1
Output: parent_1.f2, parent_1.tableoid, parent_1.ctid
-> Seq Scan on public.child1 parent_2
Output: parent_2.f2, parent_2.tableoid, parent_2.ctid
-> Seq Scan on public.child2 parent_3
Output: parent_3.f2, parent_3.tableoid, parent_3.ctid
-> Seq Scan on public.child3 parent_4
Output: parent_4.f2, parent_4.tableoid, parent_4.ctid
That is, instead of shoving the "f2 + 1" computation down to the table
scans, it gets done in a separate Result node, implying yet another
extra node in the plan with resultant slowdown. The reason for this
seems to be that apply_scanjoin_target_to_paths has special logic
to push the target down to members of a partitioned table, but it
doesn't do that for other sorts of appendrels. That isn't new
with this patch, you can see the same behavior in SELECT.
Given the distinct whiff of second-class citizenship that traditional
inheritance has today, I'm not sure how excited people will be about
fixing this. I've complained before that apply_scanjoin_target_to_paths
is brute-force and needs to be rewritten, but I don't really want to
undertake that task right now.
regards, tom lane
