Re: Parallel Append implementation

>
> + if (rel->partial_pathlist != NIL &&
> + (Path *) linitial(rel->partial_pathlist) == subpath)
> + partial_subplans_set = bms_add_member(partial_subplans_set, i);
>
> This seems like a scary way to figure this out. What if we wanted to
> build a parallel append subpath with some path other than the
> cheapest, for some reason? I think you ought to record the decision
> that set_append_rel_pathlist makes about whether to use a partial path
> or a parallel-safe path, and then just copy it over here.
>

I agree that assuming that a subpath is non-partial path if it's not
cheapest of the partial paths is risky. In fact, we can not assume
that even when it's not one of the partial_paths since it could have
been kicked out or was never added to the partial path list like
reparameterized path. But if we have to save the information about
which of the subpaths are partial paths and which are not in
AppendPath, it would take some memory, noticeable for thousands of
partitions, which will leak if the path doesn't make into the
rel->pathlist. The purpose of that information is to make sure that we
allocate only one worker to that plan. I suggested that we use
path->parallel_workers for the same, but it seems that's not
guaranteed to be reliable. The reasons were discussed upthread. Is
there any way to infer whether we can allocate more than one workers
to a plan by looking at the corresponding path?

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company