Re: GUC for cleanup indexes threshold.

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).

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center