On Thu, Feb 25, 2010 at 8:09 PM, Gokulakannan Somasundaram
<gokul007@gmail.com> wrote:
> I think, somewhere things have been misunderstood. we only need 8
> bytes more per index entry. I thought Postgres has a 8 byte transaction id,
> but it is only 4 bytes, so we only need to save the insertion and deletion
> xids. So 8 bytes more per tuple.
>
Well in the heap we need
4 bytes: xmin
4 bytes: xmax
4 bytes: cid
6 bytes: ctid
6 bytes: various info bits including natts
In indexes we currently get away with a reduced header which has few
of the 6 bytes of info bits. However the only reason we can do is
because we impose arbitrary limitations that work for indexes but
wouldn't be reasonable for tables. Such as a lower maximum number of
columns, inability to add new columns or drop columns later, etc.
--
greg
