Re: Freeze avoidance of very large table.

On Thu, Apr 23, 2015 at 09:45:38AM -0400, Robert Haas wrote:
> > Right.  My point is that either you do X 2M times to maintain that fork
> > and the overhead of the file existence, or you do one VACUUM FREEZE.  I
> > am saying that 2M is a large number and adding all those X's might
> > exceed the cost of a VACUUM FREEZE.
> 
> I agree, but if we instead make this part of the visibility map
> instead of a separate fork, the cost is much less.  It won't be any
> more expensive to clear 2 consecutive bits any time a page is touched
> than it is to clear 1.  The VM fork will be twice as large, but still
> tiny.  And the fact that you'll have only half as many pages mapping
> to the same VM page may even improve performance in some cases by
> reducing contention.  Even when it reduces performance, I think the
> impact will be so tiny as not to be worth caring about.

Agreed, no extra file, and the same write volume as currently.  It would
also match pg_clog, which uses two bits per transaction --- maybe we can
reuse some of that code.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + Everyone has their own god. +