Re: Is index enough to do simple fetch, or table is always used too?

On Sun, Dec 14, 2008 at 3:36 PM, David Rowley <dgrowley(at)gmail(dot)com> wrote:

> 2008/12/14 Dmitry Koterov <dmitry(at)koterov(dot)ru>:
> > The question: if the table "tbl" scanned to fetch "id" and calculate
> > md5(id), or the value of "id" is brought directly from "idx" index with
> no
> > table data access at all? The second behaviour is logical: why should we
> > access the table if all the needed data is already in the index entry...
>
> In fact not all the required information is in the index. Postgresql
> uses Multi-version-concurrency-control, which means there may be multi
> versions of the same row. Postgresql must hit the heap (table) no get
> the visibility information.
>

But isn't an index data is also multi-version?
If no, how could a single-versioned index be used to fetch the data from a
past snapshot?