"Kevin Grittner" <[EMAIL PROTECTED]> writes:
> Tom Lane <[EMAIL PROTECTED]> wrote:
>> You wouldn't take a new snapshot.  The thought that occurs to me is that
>> there's no reason that a transaction has to have an XID for itself
>> before it takes a snapshot.  We always special-case our own XID anyway.

> I'm having trouble picturing how that would work with a transaction using
> the SERIALIZABLE transaction isolation level,

Why?  You take a snapshot, you use it.  If you later need to allocate an
XID for yourself, you do that --- your own XID is surely uncommitted in
any case, so this doesn't affect the validity of the snapshot.

The bottom line here is that we need own-XID-if-any to be >= snapshot
xmin, but there's no obvious reason why it has to be < snapshot xmax.
This is, in fact, *not* the case for subtransaction XIDs, and I see
no fundamental reason why it need be true for the top transaction XID.

                        regards, tom lane

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