Re: pg_dump with both --serializable-deferrable and -j

Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:

>>> Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>>>
>>>> Could we start snapshot-importing transaction with repeatable
>>>> read isolation level?

>> If you are talking about having pg_dump acquire a safe snapshot and
>> have cooperating processes in the same pg_dump run use that
>> snapshot in repeatable read transactions, then yes -- that would
>> work. As long as a repeatable read transaction is using a safe
>> snapshot it will not see any anomalies.

> Why could it be unsafe to import a snapshot that's been generated as
> serializable deferrable into another backend?

That wouldn't be unsafe, which is what I was saying the post you
responded to.

> Doesn't the fact that it
> has been exported out of a deferrable xact that's still running pretty
> much guarantee that the other xact is also safe?

As long as it is at least repeatable read, yes.

> ISTM that the check is just overzelous and/or needs to be moved into
> ImportSnapshot(). There it then could be made to check if the exporting
> xact was also deferrable.

That would be great if ImportSnapshot had access to that
information; I don't see it, though. Having pg_dump use repeatable
read transactions for the processes that import the snapshot would
work fine, as long as they are reading a snapshot which was
captured by a serializable read only deferrable transaction.

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