Re: Prepared transactions vs novice DBAs, again

On Wed, Apr 22, 2009 at 5:44 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Wed, Apr 22, 2009 at 2:58 PM, Heikki Linnakangas
>> <heikki.linnakangas@enterprisedb.com> wrote:
>>> It still does. A prepared xact is just like a idle-in-transaction backend as
>>> far as vacuum is concerned.
>
>> Is that really necessary? It's true that you can't vacuum away any
>> rows whose xmin is that of the prepared xact, but it seems like you
>> wouldn't need to keep rows just because they were *visible* to the
>> prepared xact.  Once prepared, it's no longer capable of reading them.
>
> I think we've already milked what we can from that, since a prepared
> xact is treated exactly like an open one with no snapshot.  The point
> is that whatever rows it's written are still in-doubt and cannot be
> frozen, so the wraparound horizon cannot advance past its XID.

But surely that's not "the same" as a backend which is
idle-in-transaction?  In that case I think you still need a snapshot?

...Robert