On Wed, 14 May 2008, Alvaro Herrera wrote:
If neither of the bits is set, then the transaction is either in progress (which you can check by examining the list of running transactions in shared memory) or your process is the first one to check (in which case, you need to consult pg_clog to know the status, and you can update the hint bits if you find out a permanent state).
So is vacuum helpful here because it will force all that to happen in one batch? To put that another way: if I've run a manual vacuum, is it true that it will have updated all the hint bits to XMIN_COMMITTED for all the tuples that were all done when the vacuum started?
Regarding FAQs, I'm having trouble imagining putting this in the user FAQ; I think it belongs into the developer's FAQ. However, a benchmarker is not going to look there. Maybe we should start "a benchmarker's FAQ"?
On the wiki I've started adding a series of things that are performance-related FAQs. There's three of them mixed in the bottom of http://wiki.postgresql.org/wiki/Frequently_Asked_Questions right now, about slow count(*) and dealing with slow queries.
Here the FAQ would be "Why am I seeing all these writes when I'm just doing selects on my table?", and if it's mixed in with a lot of other performance related notes people should be able to find it. The answer and suggestions should be simple enough to be useful to a user who just noticed this behavior, while perhaps going into developer land for those who want to know more about the internals.
-- * Greg Smith gsmith@xxxxxxxxxxxxx http://www.gregsmith.com Baltimore, MD
