On 1/26/22 19:14, Alvaro Herrera wrote:
> On 2022-Jan-26, Robert Haas wrote:
>
>> On Tue, Jan 25, 2022 at 10:12 PM Tomas Vondra
>> <tomas.vondra@enterprisedb.com> wrote:
>
>>> 2) brin_summarize_range()
>>>
>>> Now, the issue I think is more serious, more likely to happen, and
>>> harder to fix. When summarizing a range, we write two WAL records:
>>>
>>> INSERT heapBlk 2 pagesPerRange 2 offnum 2, blkref #0: rel 1663/63 ...
>>> SAMEPAGE_UPDATE offnum 2, blkref #0: rel 1663/63341/73957 blk 2
>>>
>>> So, what happens if we lost the second WAL record, e.g. due to a crash?
>>
>> Ouch. As you say, XLogFlush() won't fix that. The WAL logging scheme
>> needs to be redesigned.
>
> I'm not sure what a good fix is. I was thinking that maybe if a
> placeholder tuple is found during index scan, and the corresponding
> process is no longer running, then the index scanner would remove the
> placeholder tuple, making the range unsummarized again. However, how
> would we know that the process is gone?
>
> Another idea is to use WAL's rm_cleanup: during replay, remember that a
> placeholder tuple was seen, then remove the info if we see an update
> from the originating process that replaces the placeholder tuple with a
> real one; at cleanup time, if the list of remembered placeholder tuples
> is nonempty, remove them.
>
> (I vaguely recall we used the WAL rm_cleanup mechanism for something
> like this, but we no longer do AFAICS.)
>
> ... Oh, but if there is a promotion involved, we may end up with a
> placeholder insertion before the promotion and the update afterwards.
> That would probably not be handled well.
>
Right, I think we'd miss those. And can't that happen for regular
recovery too. If the placeholder tuple is before the redo LSN, we'd miss
it too, right? But something prevents that.
I think the root cause is the two WAL records are not linked together,
so we fail to ensure the atomicity of the operation. Trying to fix this
by tracking placeholder tuples seems like solving the symptoms.
The easiest solution would be to link the records by XID, but of course
that goes against the whole placeholder tuple idea - no one could modify
the placeholder tuple in between. Maybe that's a price we have to pay.
>
> One thing not completely clear to me is whether this only affects
> placeholder tuples. Is it possible to have this problem with regular
> BRIN tuples? I think it isn't.
>
Yeah. I've been playing with this for a while, trying to cause issues
with non-placeholder tuples. But I think that's fine - once the tuple
becomes non-placeholder, all subsequent updates are part of the
transaction that modifies the table. So if that fails, we don't update
the index tuple.
regards
--
Tomas Vondra
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