Re: Optimization for updating foreign tables in Postgres FDW

On 2015/04/17 10:23, Amit Langote wrote:
> By the way, one suggestion may be to attach a "(pushed down)" to the
> ModifyTable's "Foreign Update". And in that case, there would be no mention of
> corresponding scan node in the list below exactly because there would be none.
> 
> postgres=# explain verbose update parent set c1 = c1;
>                                    QUERY PLAN
> ------------------------------------------------------------------------------
>   Update on public.parent  (cost=0.00..364.54 rows=4819 width=10)
>     Update on public.parent
>     Foreign Update (pushed down) on public.ft1
>     Foreign Update (pushed down) on public.ft2
>     ->  Seq Scan on public.parent  (cost=0.00..0.00 rows=1 width=10)
>           Output: parent.c1, parent.ctid

Thanks for the suggestion!

I'm not sure that that is a good idea because (1) that is contrary to
the reality (the update pushdown patch lets the ForeignScan nodes do
UPDATE/DELETE RETURNING and then do nothing at ModifyTable!) and because
(2) that might cause the problem of associating subplans' update
information with subplans' scan information, pointed out by Tom [1].

Best regards,
Etsuro Fujita

[1] http://www.postgresql.org/message-id/22505.1426986174@sss.pgh.pa.us