From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Amit Langote <amitlangote09(at)gmail(dot)com> |
Cc: | David Steele <david(at)pgmasters(dot)net>, Pavel Biryukov <79166341370(at)yandex(dot)ru>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org> |
Subject: | Re: posgres 12 bug (partitioned table) |
Date: | 2021-04-22 18:21:05 |
Message-ID: | 1102101.1619115665@sss.pgh.pa.us |
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Amit Langote <amitlangote09(at)gmail(dot)com> writes:
> On Thu, Apr 22, 2021 at 4:10 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> In the meantime, if the back branches fail with something like
>> "virtual tuple table slot does not have system attributes" when
>> trying to do this, that's not great but I'm not sure we should
>> be putting effort into improving it.
> Got it. That sounds like an acceptable compromise.
Hm, we're not done with this. Whatever you think of the merits of
throwing an implementation-level error, what's actually happening
in v12 and v13 in the wake of a71cfc56b is that an attempt to do
"RETURNING xmin" works in a non-cross-partition UPDATE, but in
a cross-partition UPDATE it dumps core. What I'm seeing is that
the result of the execute_attr_map_slot() call looks like
(gdb) p *(BufferHeapTupleTableSlot*) scanslot
$3 = {base = {base = {type = T_TupleTableSlot, tts_flags = 8, tts_nvalid = 3,
tts_ops = 0xa305c0 <TTSOpsBufferHeapTuple>,
tts_tupleDescriptor = 0x7f1c7798f920, tts_values = 0x2077100,
tts_isnull = 0x2075fb0, tts_mcxt = 0x2015ea0, tts_tid = {ip_blkid = {
bi_hi = 0, bi_lo = 0}, ip_posid = 3}, tts_tableOid = 37812},
tuple = 0x0, off = 0, tupdata = {t_len = 0, t_self = {ip_blkid = {
bi_hi = 0, bi_lo = 0}, ip_posid = 0}, t_tableOid = 0,
t_data = 0x0}}, buffer = 0}
and since base.tuple is NULL, heap_getsysattr dies on either
"Assert(tup)" or a null-pointer dereference.
So
(1) It seems like this is exposing a shortcoming in the
multiple-slot-types logic. It's not hard to understand why the slot would
look like this after execute_attr_map_slot does ExecStoreVirtualTuple,
but if this is not a legal state for a BufferHeapTuple slot, why didn't
ExecStoreVirtualTuple raise a complaint?
(2) It also seems like we can't use the srcSlot if we want to have
the fail-because-its-a-virtual-tuple behavior. I experimented with
doing ExecMaterializeSlot on the result of execute_attr_map_slot,
and that stops the crash, but then we're returning garbage values
of xmin etc, which does not seem good.
regards, tom lane
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