On Sat, 7 Jan 2006, Greg Stark wrote:
>
> "Qingqing Zhou" <zhouqq@cs.toronto.edu> writes:
>
> > For b1, it actually doesn't matter much though. With bitmap we definitely
> > can give a better EXPLAIN numbers for seqscan only, but without the bitmap,
> > we seldom make wrong choice of choosing/not choosing sequential scan.
>
> I think you have a more severe problem than that.
>
> It's not sequential scans that we have trouble estimating.
> It's the index scans that are the problem.
Exactly, we are saying the same thing.
>
> In other words, the difference between being in Postgres's buffer cache and
> being in the filesystem cache, while not insignificant, isn't really relevant
> to the planner since it affects sequential scans and index scans equally.
The bitmap was proposed since I think it is time to use dominated
shared_buffer size. Thus, if it is not in buffer cache, it is not in OS
cache either.
Regards,
Qingqing