On Thu, Dec 13, 2012 at 03:31:06PM +0000, Peter Geoghegan wrote: > On 13 December 2012 03:51, Tom Lane <t...@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?
I am looking back at this issue now and I think you are correct. The commit you mention (Oct 7 2011) says ANALYZE updates the visibility map, and the code matches that: if (!inh) vac_update_relstats(onerel, RelationGetNumberOfBlocks(onerel), totalrows, --> visibilitymap_count(onerel), hasindex, InvalidTransactionId); so if an index scan was not being used after an ANALYZE, it isn't a bad allvisibile estimate but something else. This code was in PG 9.2. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers