Re: pg_export_snapshot on standby side

On Sat, May 25, 2013 at 6:18 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 21 May 2013 19:16, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
>> We cannot run parallel pg_dump on the standby server because
>> pg_export_snapshot() always fails on the standby. Is this the oversight
>> of parallel pg_dump or pg_export_snapshot()?
>>
>> pg_export_snapshot() fails in the standby because it always assigns
>> new XID and which is not allowed in the standby. Do we really need
>> to assign new XID even in the standby for the exportable snapshot?
>
> Having looked at the code, I say No, we don't *need* to.

Good to hear.

> There are various parts of the code that deal with
> takenDuringRecovery, so much of this was clearly intended to work in
> recovery.
>
> We use the topXid for the name of the snapshot file. That is clearly
> unnecessary and we should be using the virtualxid instead like we do
> elsewhere. We also use the topXid to test whether it is still running,
> though again, we could equally use the virtualxid instead. There is no
> problem with virtualxids possibly not being active anymore, since if
> we didn't have an xid before and don't have one now, and the xmin is
> the same, the snapshot is still valid.
>
> I think we should treat this as a bug and fix it in 9.3 while we're
> still in beta.

+1

> Why? Because once we begin using the topXid as the
> filename we can't then change later to using the vxid.

I'm afraid that pg_export_snapshot() in 9.2 has already been using
topXid as the filename.

Regards,

--
Fujii Masao