Re: posgres 12 bug (partitioned table)

Hi Amit,

Thanks for your reply!

On Tue, Jul 7, 2020 at 7:18 AM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
>
> Hi Soumyadeep,
>
> Thanks for picking this up.
>
> On Tue, Jul 7, 2020 at 7:46 AM Soumyadeep Chakraborty
> <soumyadeep2007(at)gmail(dot)com> wrote:
> > Upon investigation, it seems that the problem is caused by the
> > following:
> >
> > The slot passed to the call to ExecProcessReturning() inside
> > ExecInsert() is often a virtual tuple table slot.
>
> Actually, not that often in practice. The slot is not virtual, for
> example, when inserting into a regular non-partitioned table.

Indeed! I meant partitioned tables are a common use case. Sorry, I
should have elaborated.

> If I change this to return a "heap" slot for partitioned tables, just
> like for foreign tables, the problem goes away (see the attached). In
> fact, even make check-world passes, so I don't know why it isn't that
> way to begin with.
>

This is what I had thought of initially but I had taken a step back for 2
reasons:

1. It is not mandatory for an AM to supply a heap tuple in the slot
passed to any AM routine. With your patch, the slot passed to
table_tuple_insert() inside ExecInsert() for instance is now expected to
supply a heap tuple for the subsequent call to ExecProcessReturning().
This can lead to garbage values for xmin, xmax, cmin and cmax. I tried
applying your patch on Zedstore [1], a columnar AM, and consequently, I
got incorrect values for xmin, xmax etc with the query reported in this
issue.

2. This is a secondary aspect but I will mention it here for
completeness. Not knowing enough about this code: will demanding heap
tuples for partitioned tables all throughout the code have a performance
impact? At a first glance it didn't seem to be the case. However, I did
find lots of callsites for partitioning or otherwise where we kind of
expect a virtual tuple table slot (as evidenced with the calls to
ExecStoreVirtualTuple()). With your patch, we seem to be calling
ExecStoreVirtualTuple() on a heap tuple table slot, in various places:
such as inside execute_attr_map_slot(). It seems to be harmless to do so
however, in accordance with my limited investigation.

All in all, I think we have to explicitly supply those system columns. I
heard from Daniel that one of the motivations for having table AMs
was to ensure that transaction meta-data storage is not demanded off any
AM.

[1] https://github.com/greenplum-db/postgres/tree/zedstore

Regards,
Soumyadeep