operations != passes. If you were clever, you could probably write a
modified bubble-sort algorithm that only made 2 passes. A pass is a
disk scan, operations are then performed (hopefully in memory) on what
you read from the disk. So there's no theoretical log N lower-bound on
the number of disk passes.
Not that I have anything else useful to add to this discussion, just a
tidbit I remembered from my CS classes back in college :)
-- Mark
On Fri, 2005-09-23 at 13:17 -0400, Tom Lane wrote:
> Ron Peacetree <rjpeace@earthlink.net> writes:
> > 2= No optimal external sorting algorithm should use more than 2 passes.
> > 3= Optimal external sorting algorithms should use 1 pass if at all possible.
>
> A comparison-based sort must use at least N log N operations, so it
> would appear to me that if you haven't got approximately log N passes
> then your algorithm doesn't work.
>
> regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: In versions below 8.0, the planner will ignore your desire to
> choose an index scan if your joining column's datatypes do not
> match
