Re: FailedAssertion on partprune

David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
>> It's probably best discussed on the other thread, but it seems to be
>> by design in accumulate_append_subpath(). Parallel Append nodes don't
>> get flattened if they contain a mix of parallel aware and non-parallel
>> aware subplans.

I don't really understand the issue at hand, but let me just make a
comment about accumulate_append_subpath(). If we have a regular
Append on top of another regular Append or on top of a MergeAppend, we
can flatten the lower Merge(Append) into the upper one. No problem.
If, however, the lower path is a Parallel Append, we can't. Consider
this plan:

Gather
-> Append
-> Partial Subpath #1
-> Parallel Append
-> Partial Subpath #2
-> Non-partial Subpath

Making Partial Subpath #2 a child of the Append rather than the
Parallel Append would be correct; the only downside is that we'd lose
the nice worker-balancing stuff that the Parallel Append does.
However, making Non-partial Subpath a child of the Append rather than
the Parallel Append would be dead wrong, because the Parallel Append
will make sure that it only gets executed by one of the cooperating
processes, and the regular Append won't. If the upper Append were
changed to a Parallel Append, then we could flatten the whole thing
just fine, as long as we're careful to keep partial paths and
non-partial paths separated. Also, if the situation is reversed, with
an upper Parallel Append and a regular Append beneath it, that's
always flatten-able.

> Yeah, looking at the explain posted upthread, the issue is specifically
> that some of the child paths might be for Gathers over subsets of the
> partitioning hierarchy. It's not real clear to me whether such a subset
> would necessarily correspond to a subtree of the hierarchy, but if it
> does, you'd want to be able to apply run-time pruning at the parent
> Append, in hopes of not having to execute the Gather at all. Separately
> from that, if you do execute the Gather, applying runtime pruning within
> its child Append is also a good thing.

That sounds right to me. If I'm not mistaken, in that EXPLAIN, the
radicado2012 and radicado2017 partitions are ineligible for parallel
query for some reason, maybe a reloption setting, but the other
partitions are all eligible, or rather, their subpartitions are
eligible. If they were all OK for parallel query, the plan would
probably just end up with Gather over Parallel Append over individual
sample scans.

In general, I think the only ways to get an Append are inheritance,
partitioning, and set operations. If an upper Append is due to
partitioning or inheritance, then I think that any (Merge)Append nodes
beneath it must be a subtree of the partitioning or inheritance
hierarchy, respectively. Partitioning and inheritance can't be mixed,
and set operations can't occur below partition expansion. However, if
the upper Append was created by a set operation, then its child Append
nodes could be from any of those causes. For instance, it seems
possible to imagine an upper Append that is implementing EXCEPT and a
lower Append that is implementing both partition expansion and UNION
ALL.

--
Robert Haas
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