Re: Vacuum only with 20% old tuples

On Tue, 11 Jul 2000, Tom Lane wrote:

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I suggest that we change vacuum to only move remove tuples if there is
> > more than 20% expired tuples.
> 
> > When we do vacuum, we drop all indexes and recreate them.  
> 
> > This fixes the complaint about vacuum slowness when there are many
> > expired rows in the table.  We know this is causes by excessive index
> > updates.  It allows indexes to shrink (Jan pointed this out to me.)  And
> > it fixes the TOAST problem with TOAST values in indexes.
> 
> We can't "drop and recreate" without a solution to the relation
> versioning issue (unless you are prepared to accept a nonfunctional
> database after a failure partway through index rebuild on a system
> table).  I think we should do this, but it's not all that simple...
> 
> I do not see what your 20% idea has to do with this, though, nor
> why it's a good idea.  If I've told the thing to vacuum I think
> it should vacuum.  20% of a big table could be a lot of megabytes,
> and I don't want some arbitrary decision in the code about whether
> I can reclaim that space or not.

I wouldn't mind seeing some automagic vacuum happen *if* >20% expired
... but don't understand the limit when I tell it to vacuum either ...