On Mar 25, 2011, at 9:52 AM, Merlin Moncure wrote:
> Without this bit, the only way to set hint bits going during bufmgr
> eviction is to do a visibility check on every tuple, which would
> probably be prohibitively expensive. Since OLTP environments would
> rarely see this bit, they would not have to pay for the check.
IIRC one of the biggest costs is accessing the CLOG, but what if the bufmgr.c/bgwriter didn't use the same CLOG lookup
mechanismas backends did? Unlike when a backend is inspecting visibility, it's not necessary for something like
bgwriterto know exact visibility as long as it doesn't mark something as visible when it shouldn't. If it uses a
differentCLOG caching/accessing method that lags behind the real CLOG then the worst-case scenario is that there's a
delayon setting hint bits. But getting grwiter to dothis would likely still be a huge win over forcing backends to
worryabout it. It's also possible that the visibility check itself could be simplified.
BTW, I don't think you want to play these games when a backend is evicting a page because you'll be slowing a real
backenddown.
--
Jim C. Nasby, Database Architect jim@nasby.net
512.569.9461 (cell) http://jim.nasby.net
