Re: On the usefulness of hint bits

Leonardo Francalanci <m_lists(at)yahoo(dot)it> writes:
> I was wondering what is the advantage of having hint bits for OLAP
> -style workloads, that is when the number of transactions is not
> that high.

> If I got it right, in 10 pg_clog pages we can store the status for more
> than 320000 transactions. That's a lot, in a very small space
> (80KB?).

> So I was wondering what's the gain we get from hint bits in cases
> where pg_clog is "small" (that is, will be cached by postgresql/the
> OS).

Reduction of contention for pg_clog access, for one thing. If you read
the archives, you'll find that pg_clog access contention has been shown
to be one cause of "context swap storms". Having to go to clog for
every single tuple access would make that orders of magnitude worse.

More generally, we're not going to give up hint bits even if there are
identifiable workloads where they don't buy much --- because there are
many others where they do.

regards, tom lane