Re: GUC for cleanup indexes threshold.

On Thu, Mar 9, 2017 at 10:21 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> On Wed, Mar 8, 2017 at 1:43 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>> On Sat, Mar 4, 2017 at 1:30 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>> While I can't see this explained anywhere, I'm
>>>> pretty sure that that's supposed to be impossible, which this patch
>>>> changes.
>>>>
>>>
>>> What makes you think that patch will allow pg_class.relfrozenxid to be
>>> advanced past opaque->btpo.xact which was previously not possible?
>>
>> By not reliably recycling pages in a timely manner, we won't change
>> anything about the dead page. It just sticks around. This is mostly
>> fine, but we still need VACUUM to be able to reason about it (to
>> determine if it is in fact recyclable), which is what I'm concerned
>> about breaking here. It still needs to be *possible* to recycle any
>> recyclable page at some point (e.g., when we find it convenient).
>>
>> pg_class.relfrozenxid is InvalidTransactionId for indexes because
>> indexes generally don't store XIDs. This is the one exception that I'm
>> aware of, presumably justified by the fact that it's only for
>> recyclable pages anyway, and those are currently *guaranteed* to get
>> recycled quickly. In particular, they're guaranteed to get recycled by
>> the next VACUUM. They may be recycled in the course of anti-wraparound
>> VACUUM, even if VACUUM has no garbage tuples to kill (even if we only
>> do lazy_cleanup_index() instead of lazy_vacuum_index()). This is the
>> case that this patch proposes to have us skip touching indexes for.
>>
>
> To prevent this, I think we need to not skip the lazy_cleanup_index
> when ant-wraparound vacuum (aggressive = true) even if the number of
> scanned pages is less then the threshold. This can ensure that
> pg_class.relfrozenxid is not advanced past opaque->bp.xact with
> minimal pain. Even if the btree dead page is leaved, the subsequent
> modification makes garbage on heap and then autovauum recycles that
> page while index vacuuming(lazy_index_vacuum).
>

What about if somebody does manual vacuum and there are no garbage
tuples to clean, won't in that case also you want to avoid skipping
the lazy_cleanup_index? Another option could be to skip updating the
relfrozenxid if we have skipped the index cleanup.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com