On May 29, 2017 12:21:50 PM PDT, Petr Jelinek <petr.jeli...@2ndquadrant.com> 
>On 29/05/17 20:59, Andres Freund wrote:
>> On May 29, 2017 11:58:05 AM PDT, Petr Jelinek
><petr.jeli...@2ndquadrant.com> wrote:
>>> On 27/05/17 17:17, Andres Freund wrote:
>>>> On May 27, 2017 9:48:22 AM EDT, Petr Jelinek
>>> <petr.jeli...@2ndquadrant.com> wrote:
>>>>> Actually, I guess it's the pid 47457 (COPY process) who is
>>>>> running the xid 73322726. In that case that's the same thing
>>> Masahiko
>>>>> Sawada reported [1]. Which basically is result of snapshot builder
>>>>> waiting for transaction to finish, that's normal if there is a
>>>>> transaction running when the snapshot is being created (and the
>>> is
>>>>> a long transaction).
>>>> Hm.  I suspect the issue is that the exported snapshot needs an xid
>>> for some crosscheck, and that's what we're waiting for.  Could you
>>> check what happens if you don't assign one and just content the
>>> checks out?   Not at my computer, just theorizing.
>>> I don't think that's it, in my opinion it's the parallelization of
>>> table
>>> data copy where we create snapshot for one process but then the next
>>> one
>>> has to wait for the first one to finish. Before we fixed the
>>> snapshotting, the second one would just use the ondisk snapshot so
>>> would work fine (except the snapshot was corrupted of course). I
>>> if we could somehow give it a hint to ignore the read-only txes, but
>>> then we have no way to enforce the txes to stay read-only so it does
>>> not
>>> seem safe.
>> Read-only txs have no xid ...
>That's what I mean by hinting, normally they don't but building initial
>snapshot in snapshot builder calls GetTopTransactionId() (see
>SnapBuildInitialSnapshot()) which will assign it xid.

That's precisely what I pointed out a few emails above, and what I suggest 

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