Re: TODO item: adding VERBOSE option to CLUSTER [with patch]

On Mon, Oct 13, 2008 at 8:30 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> > No, I was thinking of something along the lines of:
> > INFO: clustering "public.my_c"
> > INFO: complete, was 33%, now 100% clustered
> > The only such measure that we have is the correlation, which isn't very
> > good anyway, so I'm not sure if that's worthwhile.
>
> It'd be possible to count the number of order reversals during the
> indexscan, ie the number of tuples with CTID lower than the previous
> one's. But I'm not sure how useful that number really is. Also it's
> not clear how to preserve such functionality if cluster is
> re-implemented with a sort.
>
> regards, tom lane
>

Another version of the patch should be attached, this time counting the
number of "inversions" (pairs of tuples in the table that are in the wrong
order) as a measure of the "sortedness" of the original data (scanned/live
numbers still reported as an indication of the extent to which the table was
vacuumed).

N.B. -- I'm not familiar enough with Postgres internals to know if the
included inversion_count() method is a valid way to identify inversions.

In any case, example VERBOSE output:

postgres=# CLUSTER public.my_c VERBOSE ;
INFO: clustering "public.my_c"
INFO: complete, 15 tuples scanned, 10 tuples now live, 2 inversions
DETAIL: CPU 0.00s/0.00u sec elapsed 0.00 sec.
CLUSTER