On Sat, Jan 14, 2012 at 1:02 AM, Noah Misch <noah@leadboat.com> wrote:
>> Also, what happens if an all-visible bit gets set on the standby
>> through some other mechanism - e.g. restored from an FPI or
>> XLOG_HEAP_NEWPAGE? I'm not sure whether we ever do an FPI of the
>> visibility map page itself, but we certainly do it for the heap pages.
>> So it might be that this infrastructure would (somewhat bizarrely)
>> trust the visibility map bits but not the PD_ALL_VISIBLE bits.
>
> Simon spoke to the FPI side of the question. For heap pages, the
> XLOG_HEAP_NEWPAGE consumers are CLUSTER, VACUUM FULL and ALTER TABLE SET
> TABLESPACE. For the last, we will have already logged any PD_ALL_VISIBLE bits
> through normal channels. CLUSTER and VACUUM FULL never set PD_ALL_VISIBLE or
> visibility map bits. When, someday, they do, we might emit a separate WAL
> record to force the recovery conflict. However, CLUSTER/VACUUM FULL already
> remove tuples still-visible to standby snapshots without provoking a recovery
> conflict. (Again only with hot_standby_feedback=off.)
If that were the case it would be a bug.
CLUSTER/VACUUM FULL emit an AccessExclusiveLock record that would
conflict with any current lock holders, so should be fine on that.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
