On Aug 13, 2011, at 4:31 PM, Heikki Linnakangas wrote:
>> The example is much more realistic if the query is a fetch of N latest rows 
>> from a table. Very common use case, and the whole relation's visibility 
>> statistics are completely wrong for that query.
> That is somewhat compensated by the fact that tuples that are accessed more 
> often are also more likely to be in cache. Fetching the heap tuple to check 
> visibility is very cheap when the tuple is in cache.
> I'm not sure how far that compensates it, though. I'm sure there's typically 
> nevertheless a fairly wide range of pages that have been modified since the 
> last vacuum, but not in cache anymore.

http://xkcd.org/937/ :)

Could something be added to pg_stats that tracks visibility map usefulness on a 
per-attribute basis? Perhaps another set of stats buckets that show visibility 
percentages for each stats bucket?
Jim C. Nasby, Database Architect                   j...@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net

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