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

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.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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