Re: collateral benefits of a crash-safe visibility map

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, May 10, 2011 at 12:57 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> Hmmm, do we really need to WAL log freezing?

> That might solve the relfrozenxid problem - set the bits in the heap,
> sync the heap, then update relfrozenxid once the heap is guaranteed
> safely on disk - but it again seems problematic for Hot Standby.

... or even warm standby.  You basically *have to* WAL-log freezing
before you can truncate pg_clog.  The only freedom you have here is
freedom to mess with the policy about how soon you try to truncate
pg_clog.

(Doing an unlogged freeze operation first is right out, too, if it
causes the system to fail to perform/log the operation later.)
        regards, tom lane