Re: Index Scans become Seq Scans after VACUUM ANALYSE

Curt Sampson wrote:
> 
> On Tue, 23 Apr 2002, mlw wrote:
> 
> > > On a system that has neither read-ahead nor sorting of I/O requests,
> > > yes. Which systems are you using that provide neither of these
> > > facilities?
> >
> > This only happens if the OS can organize the I/O requests in such a manner. It
> > is a non-trivial function.
> 
> Well, if you call less than 200 lines of code (including lots of
> comments), "non-trivial," yes. Have a look at NetBSD's
> src/sys/kern/subr_disk.c for one example implementation.
> 
> But trivial or not, if all operating systems on which Postgres runs
> are doing this, your point is, well, pointless. So, once again, which
> systems are you using that do *not* do this?

I am not arguing about whether or not they do it, I am saying it is not always
possible. I/O requests do not remain in queue waiting for reordering
indefinitely.