Simon Riggs <> writes:
> On Fri, 2011-01-21 at 11:19 +0200, Heikki Linnakangas wrote:
>> It's not the order in which the xid was assigned that matters, but the 
>> order the transactions started and got their snapshots. The xids might 
>> be assigned a lot later, after the transactions have already read data.

> So if a read-write transaction assigns an xid before it takes a snapshot
> then we'll be OK? That seems much easier to arrange than passing chunks
> of snapshot data backwards and forwards. Optionally.

No, that idea is DOA from a performance standpoint.  We sweated blood to
avoid having to assign XIDs to read-only transactions, and we're not
going back.  If SSI requires that, SSI is not getting committed.

                        regards, tom lane

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