Re: crash-safe visibility map, take three

On Wed, Dec 1, 2010 at 11:40 AM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> On 01.12.2010 18:25, Robert Haas wrote:
>>
>> I think we can improve this a bit further by also introducing a
>> HEAP_XMIN_FROZEN bit that we set in lieu of overwriting XMIN with
>> FrozenXID.  This allows us to freeze tuples aggressively - if we want
>> - without losing any forensic information.  We can then modify the
>> above algorithm slightly, so that when we observe that a page is all
>> visible, we not only set PD_ALL_VISIBLE on the page but also
>> HEAP_XMIN_FROZEN on each tuple.
>
> Hmm, actually, if we're willing to believe PD_ALL_VISIBLE in the page header
> over the xmin/xmax on the tuples, we could simply not bother doing
> anti-wraparound vacuums for pages that have the flag set. I'm not sure what
> changes that would require outside heapam.c, as we'd have to be careful to
> not trust the xmin/xmax if the flag was set.
>
> The first update on the page that clears the flag would need to freeze all
> the tuples in that scheme.

That seems more complex for no particular gain.  I guess it saves an
infomask bit, but I'm willing to burn one to reduce code complexity.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company