Re: Expression errors with "FOR UPDATE" and postgres_fdw with partition wise join enabled.

On Thu, Jul 26, 2018 at 2:12 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Tue, Jul 24, 2018 at 7:51 AM, Etsuro Fujita
>> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>> I'm not sure that's a good idea, because I think we have a trade-off
>>> relation; the more we make create_plan simple, the more we need to make
>>> earlier states of the planner complicated.
>>>
>>> And it looks to me like the partitionwise join code is making earlier (and
>>> later) stages of the planner too complicated, to make create_plan simple.
>
>> I think that create_plan is *supposed* to be simple. Its purpose is
>> to prune away data that's only needed during planning and add things
>> that can be computed at the last minute which are needed at execution
>> time. Making it do anything else is, in my opinion, not good.
>
> I tend to agree with Robert on this. In particular, if you put anything
> into create_plan or later that affects cost estimates, you're really doing
> it wrong, because changing those estimates might've changed the plan
> entirely. (As I recall, we have a couple of cheats like that now,
> associated with dropping no-op projection and subquery scan nodes.
> But let's not add more.)

+1

>
> TBH, I think this entire discussion is proving what sheer folly it was
> to allow partitions to have rowtypes not identical to their parent.
> But I suppose we're stuck with that now.
>

Every table created has a different rowtype and when the partition's
rowtype is different from that of the partitioned table, we add a
ConvertRowtypeExpr node on top of the whole-row expression.

2153 if (appinfo->parent_reltype != appinfo->child_reltype)
2154 {
2155 ConvertRowtypeExpr *r =
makeNode(ConvertRowtypeExpr);

We may be able to set rowtype of a partition to be same as the rowtype
of a partitioned table when a partition table is created using an
ALTER TABLE syntax. But are you suggesting that we change the reltype
of a partition being attached? What happens when we detach a partition
and let the table live independent of the partitioned table? Do we
create a new reltype?

I am not even going into the problem when we try to attach a partition
with different tuple layout. I do believe that such usecases arise and
rewriting the table being attached is a viable option.

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