Re: Synchronized Scan update

On Tue, 2007-03-13 at 12:53 -0400, Tom Lane wrote:
> Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> > I agree that ss_report_loc() doesn't need to report on every call. If
> > there's any significant overhead I agree that it should report less
> > often. Do you think that the overhead is significant on such a simple
> > function?
>
> One extra LWLock cycle per page processed definitely *is* a significant
> overhead ... can you say "context swap storm"? I'd think about doing it
> once every 100 or so pages.
>

No lock is needed to store the hint. If somehow the hint (which is
stored in a static table, no pointers) gets invalid data due to a race
condition, the new scan will simply consider the hint invalid and start
at 0.

I did this precisely to avoid causing a performance regression for usage
patterns that don't benefit from sync scans.

Regards,
Jeff Davis