Re: Visibility map, partial vacuums

On Tue, 2008-10-28 at 13:58 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > On Mon, 2008-10-27 at 14:03 +0200, Heikki Linnakangas wrote:
> >> Lazy VACUUM only needs to visit pages that are '0' in the visibility 
> >> map. This allows partial vacuums, where we only need to scan those parts 
> >> of the table that need vacuuming, plus all indexes.
> 
> > Just realised that this means we still have to visit each block of a
> > btree index with a cleanup lock.
> 
> Yes, and your proposal cannot fix that.  Read "The Deletion Algorithm"
> in nbtree/README, particularly the second paragraph.

Yes, understood. Please read the algorithm again. It does guarantee that
each block in the index has been checked to see if nobody is pinning it,
it just avoids performing I/O to prove that.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support