Re: posgres 12 bug (partitioned table)

Hi,

On 2020-06-04 12:57:30 -0400, Tom Lane wrote:
> Pavel Biryukov <79166341370(at)yandex(dot)ru> writes:
> > Hello. I'm catch error "virtual tuple table slot does not have system
> > attributes" when inserting row into partitioned table with RETURNING
> > xmin
>
> Reproduced here. The example works in v11, and in v10 if you remove
> the unnecessary-to-the-example primary key, so it seems like a clear
> regression. I didn't dig for the cause but I suppose it's related
> to Andres' slot-related changes.

The reason we're getting the failure is that nodeModifyTable.c only is
dealing with virtual tuple slots, which don't carry visibility
information. Despite actually having infrastructure for creating a
partition specific slot. If I force check_attrmap_match() to return
false, the example starts working.

I don't really know how to best fix this in the partitioning
infrastructure. Currently the determination whether there needs to be
any conversion between subplan slot and the slot used for insertion is
solely based on comparing tuple descriptors. But for better or worse, we
don't represent system column accesses in tuple descriptors.

It's not that hard to force the slot creation & use whenever there's
returning, but somehow that feels hackish (but so does plenty other
things in execPartition.c). See attached.

But I'm worried that that's not enough: What if somebody in a trigger
wants to access system columns besides tableoid and tid (which are
handled in a generic manner)? Currently - only for partitioned table DML
going through the root table - we'll not have valid values for the
trigger. It's pretty dubious imo to use xmin/xmax in triggers, but ...

I suspect we should just unconditionally use
partrouteinfo->pi_PartitionTupleSlot. Going through
partrouteinfo->pi_RootToPartitionMap if present, and ExecCopySlot()
otherwise.

Medium term I think we should just plain out forbid references to system
columns in partioned tables Or at least insist that all partitions have
that column. There's no meaningful way for some AMs to have xmin / xmax
in a compatible way with heap.

Greetings,

Andres Freund