Re: [HACKERS] UPDATE of partition key

On Fri, Jun 30, 2017 at 4:20 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> I don't think the approach of building a hash table to figure out
> which result rels have already been created is a good one.  That too
> feels like something that the planner should be figuring out and the
> executor should just be implementing what the planner decided.  I
> haven't figured out exactly how that should work yet, but it seems
> like it ought to be doable.

I was imagining when I wrote the above that the planner should somehow
compute a list of relations that it has excluded so that the executor
can skip building ResultRelInfos for exactly those relations, but on
further study, that's not particularly easy to achieve and wouldn't
really save anything anyway, because the list of OIDs is coming
straight out of the partition descriptor, so it's pretty much free.
However, I still think it would be a nifty idea if we could avoid
needing the hash table to deduplicate.  The reason we need that is, I
think, that expand_inherited_rtentry() is going to expand the
inheritance hierarchy in whatever order the scan(s) of pg_inherits
return the descendant tables, whereas the partition descriptor is
going to put them in a canonical order.

But that seems like it wouldn't be too hard to fix: let's have
expand_inherited_rtentry() expand the partitioned table in the same
order that will be used by ExecSetupPartitionTupleRouting().  That
seems pretty easy to do - just have expand_inherited_rtentry() notice
that it's got a partitioned table and call
RelationGetPartitionDispatchInfo() instead of find_all_inheritors() to
produce the list of OIDs.  Then - I think -
ExecSetupPartitionTupleRouting() doesn't need the hash table; it can
just scan through the return value of ExecSetupPartitionTupleRouting()
and the list of already-created ResultRelInfo structures in parallel -
the order must be the same, but the latter can be missing some
elements, so it can just create the missing ones.

-- 
Robert Haas
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