Re: logical replication busy-waiting on a lock

On 02/06/17 03:38, Robert Haas wrote:
> On Thu, Jun 1, 2017 at 2:28 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> On 2017-06-01 14:17:44 +0200, Petr Jelinek wrote:
>>> Thinking more about this, I am not convinced it's a good idea to change
>>> exports this late in the cycle. I still think it's best to do the xid
>>> assignment only when the snapshot is actually exported but don't assign
>>> xid when the export is only used by the local session (the new option in
>>> PG10). That's one line change which impacts only logical
>>> replication/decoding as opposed to everything else which uses exported
>>> snapshots.
>>
>> I'm not quite convinced by this argument. Exported snapshot contents
>> are ephemeral, we can change the format at any time. The wait is fairly
>> annoying for every user of logical decoding. For me the combination of
>> those two fact implies that we should rather fix this properly.
>
> +1. The change Andres is proposing doesn't sound like it will be
> terribly high-impact, and I think we'll be happier in the long run if
> we install real fixes instead of kludging it.
>

All right, here is my rough attempt on doing what Andres has suggested,
going straight from xid to vxid, seems to work fine in my tests and
parallel pg_dump seems happy with it as well.

The second patch removes the xid parameter from SnapBuildBuildSnapshot
as it's not used for anything and skips the GetTopTransactionId() call
as well. That should solve the original problem reported here.

--
Petr Jelinek http://www.2ndQuadrant.com/
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