Re: Optimizing away second VACUUM heap scan when only BRIN indexes on table

On Sun, Dec 20, 2015 at 11:14:46PM -0300, Alvaro Herrera wrote:
> Jim Nasby wrote:
> > On 11/23/15 5:06 PM, Peter Geoghegan wrote:
> > >I realize that the second scan performed by lazy_vacuum_heap() only
> > >visits those pages known to contain dead tuples. However, the
> > >experience of seeing problems with the random sampling of ANALYZE
> > >makes me think that that might not be very helpful. There is no good
> > >reason to think that there won't be a uniform distribution of dead
> > >tuples across the heap, and so only visiting pages known to contain
> > >dead tuples might be surprisingly little help even when there are
> > >relatively few VACUUM-able tuples in the table.
> >
> > Even worse is if you can't fit all the dead TIDs in memory and have to do
> > multiple passes for no reason...
>
> Since BRIN indexes cannot be primary keys nor unique keys, it's hard to
> be convinced that the use case of a table with only BRIN indexes is
> terribly interesting.

If you've got high-frequency logs, timestamptz might not operate at
fine enough a grain to form a primary key, but it's just the kind of
thing BRIN is great at narrowing down.

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
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