Re: Possible documentation error

On Tue, Dec 26, 2006 at 07:22:21PM +0100, Martijn van Oosterhout wrote:
> On Tue, Dec 26, 2006 at 12:49:55PM -0500, D'Arcy J.M. Cain wrote:
> > On Tue, 26 Dec 2006 18:12:45 +0100
> > Martijn van Oosterhout <kleptog@svana.org> wrote:
> > > On Tue, Dec 26, 2006 at 12:04:40PM -0500, D'Arcy J.M. Cain wrote:
> > > > Now it certainly seems to me that it should behave as described given
> > > > the definition of VACUUM FULL so I am a little confused by my tests.
> > > > My test table only has two entries in it.  Is that the issue?  In fact,
> > > > I find the same behaviour if I do a simple VACUUM on the table.
> > > 
> > > On a table with two entries, VACUUM FULL is going to do nothing of
> > > interest. Moving tuples within a page is useless, generally.
> > 
> > I thought that that might be the issue.  The docs should probably say
> > "can" instead of "will" then.
> 
> The doumenttion is accurate as is. It says when "moved by VACUUM FULL".
> In your case they wern't moved. If you change the word "will" to "can",
> it will be wrong.

Howso? There's no guarantee (which is what "will" implies) that a ctid
will change on VACUUM FULL. In fact, your example demonstrates that; 0,1
stayed put.

I'm sorry if it sounds like I'm picking nits, but using CTID to
identify rows could provide a noticeable performance gain in some cases.
But users can't make use of that if it's not clear exactly when and how
CTIDs can change.
-- 
Jim Nasby                                            jim@nasby.net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)