Re: Do we need so many hint bits?

On Thu, 2012-11-15 at 22:21 -0500, Tom Lane wrote:
> We aren't pressed for flag bits particularly. I think the main
> attraction of this idea is precisely to reduce unnecessary page dirties,
> and so that leads me to suggest a variant: keep the four bits defined as
> now, but do not attempt to set XMIN_INVALID or XMAX_COMMITTED unless the
> page is already dirty. This would make it a straight-up trade of more
> clog consultation for fewer page dirties.

When I tried a simple test on my laptop, where I removed the
XMIN_INVALID and XMAX_COMMITTED bits on a simple SELECT test (no aborts,
no deletes, fits in shared_buffers) I saw a pretty consistent 2%
speedup. Not earth-shattering, but it would be nice if we could figure
out a way to do that.

Not sure where that speedup came from, to be honest, it doesn't look
like much difference in the work being done in the visibility check.

Regards,
Jeff Davis