Richard Huxton <d...@archonet.com> writes: > So what is the cost of calculating the hint-bits for a whole block of > tuples in one go vs reading that block from actual spinning disk?
Potentially a couple of hundred times worse, if you're unlucky and each XID on the page requires visiting a different block of clog that's also not in memory. The average case probably isn't that bad, but I think we'd likely be talking at least a factor of two penalty --- you'd be hopelessly optimistic to assume you didn't need at least one clog visit per page. Also, if you want to assume that you're lucky and the XIDs mostly fall within a fairly recent range of clog pages, you're still not out of the woods. In that situation what you are talking about is a spectacular increase in the hit rate for cached clog pages --- which are already a known contention bottleneck in many scenarios. > While on that topic - I'm assuming freezing requires substantially more > effort than updating hint bits? It's a WAL-logged page change, so at minimum double the cost. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers