Re: postgres_fdw: Oddity in pushing down inherited UPDATE/DELETEjoins to remote servers

(2018/05/17 14:19), Amit Langote wrote:
> Looking at this for a bit, I wondered if this crash wouldn't have occurred
> if the "propagation" had also considered join relations in addition to
> simple relations.  For example, if I changed inheritance_planner like the
> attached (not proposing that we consider committing it), reported crash
> doesn't occur.  The fact that it's not currently that way means that
> somebody thought that there is no point in keeping all of those joinrels
> around until plan creation time.

One reason for that would be that we use the per-child PlannerInfo, not 
the parent one, at plan creation time.  Here is the code in 
create_modifytable_plan:

         /*
          * In an inherited UPDATE/DELETE, reference the per-child modified
          * subroot while creating Plans from Paths for the child rel. 
This is
          * a kluge, but otherwise it's too hard to ensure that Plan 
creation
          * functions (particularly in FDWs) don't depend on the contents of
          * "root" matching what they saw at Path creation time.  The main
          * downside is that creation functions for Plans that might appear
          * below a ModifyTable cannot expect to modify the contents of 
"root"
          * and have it "stick" for subsequent processing such as setrefs.c.
          * That's not great, but it seems better than the alternative.
          */
         subplan = create_plan_recurse(subroot, subpath, CP_EXACT_TLIST);

So, we don't need to accumulate the joinrel lists for child relations 
into a single list and store that list into the parent PlannerInfo in 
inheritance_planner, as in the patch you proposed.  I think the change 
by the commit is based on the same idea as that.

Best regards,
Etsuro Fujita