Re: How much do the hint bits help?

Josh Berkus <josh@agliodbs.com> writes:
> Regarding the contention which Tom expects: the extra load on the CLOG
> would be 100% reads, no?  If it's *all* reads, why would we have any
> more contention than we have now?

Read involves sharelock which still causes contention.  Those bufmgr
contention storms we saw before were completely independent of whether
the pages were accessed for read or for write.

Another thing to keep in mind is that the current clog access code is
designed on the assumption that there's considerable locality of access
to pg_clog, ie, you usually only need to consult it for recent XIDs
because older ones have been hinted.  Turn off hint bits, that behavior
goes out the window.
        regards, tom lane