Re: About "Our CLUSTER implementation is pessimal" patch

Hi all,

attached a patch to do seq scan + sorting instead of index scan

on CLUSTER (when that's supposed to be faster).

As I've already said, the patch is based on:
http://archives.postgresql.org/pgsql-hackers/2008-08/msg01371.php

Of course, the code isn't supposed to be ready to be merged: I
would like to write more comments and add some test cases to
cluster.sql (plus change all the things you are going to tell me I
have to change...)

I would like your opinions on code correctness and the decisions
I took, especially:

1) function names ("cost_index_scan_vs_seqscansort" I guess
it's awful...)

2) the fact that I put in Tuplesortstate an EState variable, so that
MakeSingleTupleTableSlot wouldn't have to be called for every
row in the expression indexes case

3) the expression index case is not "optimized": I preferred to
call FormIndexDatum once for the first key value in
copytup_rawheap and another time to get all the remaining values
in comparetup_rawheap. I liked the idea of re-using
FormIndexDatum in that case, instead of copying&pasting only
the relevant code: but FormIndexDatum returns all the values,

even when I might need only the first one

4) I refactored the code to deform and rewrite tuple into the function
"deform_and_rewrite_tuple", because now that part can be called
by the regular index scan or by the new seq-scan + sort (but I
could copy&paste those lines instead of refactoring them into a new
function)

Suggestions and comments are not just welcome, but needed!

Leonardo