Richard Huxton <dev@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
