Re: a thinko in b676ac443b6

On 7/28/21 3:15 AM, Amit Langote wrote:
> On Wed, Jul 28, 2021 at 1:07 AM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>> On 7/27/21 4:28 AM, Amit Langote wrote:
>>> I think it can be incorrect to use the same TupleDesc for both the
>>> slots in ri_Slots (for ready-to-be-inserted tuples) and ri_PlanSlots
>>> (for subplan output tuples). Especially if you consider what we did
>>> in 86dc90056df that was committed into v14. In that commit, we
>>> changed the way a subplan under ModifyTable produces its output for an
>>> UPDATE statement. Previously, it would produce a tuple matching the
>>> target table's TupleDesc exactly (plus any junk columns), but now it
>>> produces only a partial tuple containing the values for the changed
>>> columns.
>>>
>>> So it's better to revert to using planSlot->tts_tupleDescriptor for
>>> the slots in ri_PlanSlots. Attached a patch to do so.
>>
>> Yeah, this seems like a clear mistake - thanks for noticing it! Clearly
>> no regression test triggered the issue, so I wonder what's the best way
>> to test it - any idea what would the test need to do?
>
> Ah, I should've mentioned that this is only a problem if the original
> query is an UPDATE. With v14, only INSERTs can use batching and the
> subplan does output a tuple matching the target table's TupleDesc in
> their case, so the code seems to work fine.
>
> As I said, I noticed a problem when rebasing my patch to allow
> cross-partition UPDATEs to use batching for the inserts that are
> performed internally to implement such UPDATEs. The exact problem I
> noticed is that the following Assert tts_virtual_copyslot() (via
> ExecCopySlot called with an ri_PlanSlots[] entry) failed:
>
> Assert(srcdesc->natts <= dstslot->tts_tupleDescriptor->natts);
>
> srcdesc in this case is a slot in ri_PlanSlots[] initialized with the
> target table's TupleDesc (the "thinko") and dstslot is the slot that
> holds subplan's output tuple ('planSlot' passed to ExecInsert). As I
> described in my previous email, dstslot's TupleDesc can be narrower
> than the target table's TupleDesc in the case of an UPDATE, so the
> Assert can fail in theory.
>
>> I did some quick experiments with batched INSERTs with RETURNING clauses
>> and/or subplans, but I haven't succeeded in triggering the issue :-(
>
> Yeah, no way to trigger this except UPDATEs. It still seems like a
> good idea to fix this in v14.
>

OK, thanks for the explanation. So it's benign in v14, but I agree it's
better to fix it there too. I'll get this sorted/pushed.

regards

--
Tomas Vondra
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