Duplicating transaction information in indexes and performing in memory vacuum

Hi,

Last week, there was a thread whether solely in memory vacuum can be performed
or not.(OK, that was a part of thread but anyways)

I suggested that a page be vacuumed when it is pushed out of buffer cache. Tom
pointed out that it can not be done as index tuples stote heap tuple id and
depend upon heap tuple to find out transaction information.

I asked is it feasible to add transaction information to index tuple and the
answer was no.

I searched hackers archive and following is only thread I could come up in this
context.

http://archives.postgresql.org/pgsql-hackers/2000-09/msg00513.php
http://archives.postgresql.org/pgsql-hackers/2001-09/msg00409.php

The thread does not consider vacuum at all.

What are (more) reasons for not adding transaction information to index tuple,
in addition to heap tuple?

Cons are bloated indexes. The index tuple size will be close to 30 bytes minimum.

On pro* side of this, no more vacuum required (at least for part of data that is
being used. If data isn't used, it does not need vacuum anyway) and space bloat
is stopped right in memory, without incurring overhead of additional IO vacuum
demands.

Given recent trend of pushing PG higher and higher in scale (From performance
list traffic, that is), I think this could be worthwhile addition.

So what are the cons I missed so far?

Bye
Shridhar