Re: [PERFORM] encouraging index-only scans

On 13 December 2012 03:51, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> ANALYZE does not set that value, and is not going to start doing so,
> because it doesn't scan enough of the table to derive a trustworthy
> value.

I'm slightly surprised by your remarks here, because the commit
message where the relallvisible column was added (commit
a2822fb9337a21f98ac4ce850bb4145acf47ca27) says:

"Add a column pg_class.relallvisible to remember the number of pages
that were all-visible according to the visibility map as of the last
VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to
estimate how many heap page fetches can be avoided during an
index-only scan."

Have I missed some nuance?

-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services