On Mon, Aug 22, 2016 at 6:39 PM, Craig Ringer <craig@2ndquadrant.com> wrote:
> On 23 Aug 2016 05:43, "Kevin Grittner" <kgrittn@gmail.com> wrote:
>> On Mon, Aug 22, 2016 at 3:29 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>
>>> it seems to me that
>>> this is just one facet of a much more general problem: given two
>>> transactions T1 and T2, the order of replay must match the order of
>>> commit unless you can prove that there are no dependencies between
>>> them. I don't see why it matters whether the operations are sequence
>>> operations or data operations; it's just a question of whether they're
>>> modifying the same "stuff".
>> The commit order is the simplest and safest *unless* there is a
>> read-write anti-dependency a/k/a read-write dependency a/k/a
>> rw-conflict: where a read from one transaction sees the "before"
>> version of data modified by the other transaction. In such a case
>> it is necessary for correct serializable transaction behavior for
>> the transaction that read the "before" image to be replayed before
>> the write it didn't see, regardless of commit order. If you're not
>> trying to avoid serialization anomalies, it is less clear to me
>> what is best.
>
> Could you provide an example of a case where xacts replayed in
> commit order will produce incorrect results?
https://wiki.postgresql.org/wiki/SSI#Deposit_Report
... where T3 is on the replication target.
--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
