Re: Prepared transactions vs novice DBAs, again

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.
        regards, tom lane