Re: GUC for cleanup indexes threshold.

On Sat, Mar 4, 2017 at 8:55 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Fri, Mar 3, 2017 at 6:58 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> You are right that we don't want the number of unclaimed-by-FSM
>> recyclable pages to grow forever, but I think that won't happen with
>> this patch. As soon as there are more deletions (in heap), in the
>> next vacuum cycle, the pages will be reclaimed by lazy_vacuum_index().
>
> Right.
>
>>> (Thinks about it some more...)
>>>
>>> Unfortunately, I just saw a whole new problem with this patch:
>>> _bt_page_recyclable() is the one place in the B-Tree AM where we stash
>>> an XID.
>>>
>>
>> Can you be more specific to tell which code exactly you are referring here?
>
> I meant that we stash an XID when a B-Tree page is deleted, used to
> determine when it's finally safe to to recycle the page by comparing
> it to RecentGlobalXmin (recyling will happen during the next VACUUM).
> We can't immediately recycle a page, because an existing index scan
> might land on the page while following a stale pointer.
>
> _bt_page_recyclable(), which checks if recycling is safe (no possible
> index scan can land of dead page), is a pretty small function. I'm
> concerned about what happens when
> pg_class.relfrozenxid/pg_database.datfrozenxid are advanced past
> opaque->btpo.xact.
>

Are you talking pg_class.relfrozenxid of index or table? IIUC, for
indexes it will be InvalidTransactionId and for tables, I think it
will be updated with or without the patch.

> 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?

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