Re: [Xen-devel] [HACKERS] [OSSTEST PATCH 0/1] PostgreSQL db: Retry on constraint violation [and 2 more messages] [and 1 more messages]

[Kevin Grittner <kgrittn@xxxxxxxxx>]

On Fri, Dec 16, 2016 at 8:24 AM, Robert Haas <robertmhaas@xxxxxxxxx> wrote:
> On Thu, Dec 15, 2016 at 9:01 AM, Kevin Grittner <kgrittn@xxxxxxxxx> wrote:
>> I also realized some other properties of read-only transactions
>> that might interest you (and that I should probably document).
>> Since the only way for a read-only transaction to be the on
>> experiencing a serialization failure is if Tout commits before the
>> read-only transaction (which is always Tin) acquires its snapshot,
>> Tpivot is still running when Tin acquires its snapshot, Tpivot
>> commits before a serialization failure involving Tin is detected,
>> and *then* Tin reads a data set affected by the writes of Tpivot.
>> Since a snapshot is only acquired when a statement is run which
>> requires a snapshot, that means that a query run in an implicit
>> transaction (i.e., there is no START or BEGIN statement to
>> explicitly start it; the SELECT or other data-accessing statement
>> automatically starts the transaction so it has a valid context in
>> which to run) that does not write data can never return bad results
>> nor receive a serialization failure.  Nor can those things happen
>> on the *first* or *only* non-writing statement in an explicit
>> transaction.
>
> I don't understand this argument.  Every statement in a read-only,
> serializable transaction runs with the same snapshot, so I don't see
> how it can make a difference whether we're in the middle of running
> the first statement or the tenth.  Tpivot might commit in the middle
> of executing the first statement of the transaction, which might then
> -- later on during the execution of that same statement -- do
> something that causes it to acquire a bunch more SIREAD locks.

Good point.  For the read only transaction to be the one to receive
a serialization failure, it must acquire a snapshot while Tpivot is
still running, and read a data set which was affected by Tpivot,
and must do so after Tpivot has successfully committed.  However,
if the commit of Tpivot comes after Tin has parsed the statement,
determined that it is one that requires a snapshot, and acquired
its snapshot and before it reads the modified data set, Tin could
get the serialization failure.  Muddled thinking on my part to
think of acquiring the snapshot to be atomic with running the
statement which caused the snapshot to be acquired.

Good catch!

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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