Re: WIP: long transactions on hot standby feedback replica / proof of concept

On Fri, Nov 3, 2017 at 5:57 PM, Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> On Thu, Nov 2, 2017 at 5:56 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> > I can propose following alternative approach: teach read-only queries on
>> > hot
>> > standby to tolerate concurrent relation truncation. Therefore, when
>> > non-existent heap page is accessed on hot standby, we can know that it
>> > was
>> > deleted by concurrent truncation and should be assumed to be empty. Any
>> > thoughts?
>>
>> Sounds like it might break MVCC?
>
> I don't know why it might be broken. VACUUM truncates heap only when tail
> to be truncated is already empty. When applying truncate WAL record,
> previous WAL records deleting all those tuples in the tail are already
> applied. Thus, if even MVCC is broken and we will miss some tuples after
> heap truncation, they were anyway were gone before heap truncation.

Ah - I was thinking of the TRUNCATE command, rather than truncation by
VACUUM. Your argument makes sense, although the case where the
relation is truncated and later re-extended might need some thought.

--
Robert Haas
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