Re: I: About "Our CLUSTER implementation is pessimal" patch

Itagaki Takahiro <itagaki.takahiro@gmail.com> writes:
> I re-ordered some description in the doc. Does it look better?
> Comments and suggestions welcome.

Applied with some significant editorialization.  The biggest problem I
found was that the code for expression indexes didn't really work, and
would leak memory like there's no tomorrow even when it did work.
I fixed that, but I think the performance is still going to be pretty
undesirable.  We have to re-evaluate the index expressions for each
tuple each time we do a comparison, which means it's going to be really
really slow unless the index expressions are *very* cheap.  But perhaps
the use-case for clustering on expression indexes is small enough that
this isn't worth worrying about.

I considered computing the index expressions just once as the data is
being fed in, and including their values in the tuples-to-be-sorted;
that would cut the number of times the values have to be computed by
a factor of about log N.  But it'd also bloat the on-disk sort data,
which could possibly cost more in I/O than we save.  So it's not real
clear what to do anyway.
        regards, tom lane