Re: crash-safe visibility map, take three

On 30.11.2010 18:40, Tom Lane wrote:
> Robert Haas<robertmhaas(at)gmail(dot)com> writes:
>> That's definitely sucky, but in some ways it would be more complicated
>> if they did, because I don't think all-visible on the master implies
>> all-visible on the standby.
>
> Ouch. That seems like it could shoot down all these proposals. There
> definitely isn't any way to make VM crash-safe if there is no WAL-driven
> mechanism for setting the bits.

Note that this is only a problem for *hot* standby. After failover, all
the tuples that were visible to everyone in the master are also visible
to all new transactions in the standby.

We dealt with this in 9.0 already, with the "killed" flag in index
tuples and the PD_ALL_VISIBLE flag in heap scans. We simply don't
believe them in hot standby mode, and check visibility even if the flag
is set.

> I guess what we need is a way to delay the application of such a WAL
> record on the slave until it's safe, which means the record also has to
> carry some indication of the youngest XMIN on the page.

Something like that would certainly be nice. With index-only scans, it
can be a big disappointment if you can't do an index-only scan in hot
standby.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com