On 08/10/17 15:21, Craig Ringer wrote:
> On 8 October 2017 at 02:37, Daniele Varrazzo <daniele.varrazzo@gmail.com> wrote:
>> Hello,
>>
>> we have been reported, and I have experienced a couple of times,
>> pg_repack breaking logical replication.
>>
>> - https://github.com/reorg/pg_repack/issues/135
>> - https://github.com/2ndQuadrant/pglogical/issues/113
>
> Yeah, I was going to say I've seen reports of this with pglogical, but
> I see you've linked to them.
>
> I haven't had a chance to look into it though, and haven't had a
> suitable reproducible test case.
>
>> In the above issue #113, Petr Jelinek commented:
>>
>>> From quick look at pg_repack, the way it does table rewrite is almost guaranteed
>>> to break logical decoding unless there is zero unconsumed changes for a given table
>>> as it does not build the necessary mappings info for logical decoding that standard
>>> heap rewrite in postgres does.
>>
>> unfortunately he didn't follow up to further details requests.
>
> At a guess he's referring to src/backend/access/heap/rewriteheap.c .
>
> I'd explain better if I understood what was going on myself, but I
> haven't really understood the logical decoding parts of that code.
>
>> - Is Petr diagnosis right and freezing of logical replication is to be
>> blamed to missing mapping?
>> - Can you suggest a test to reproduce the issue reliably?
>> - What are mapped relations anyway?
>
> I can't immediately give you the answers you seek, but start by
> studying src/backend/access/heap/rewriteheap.c . Notably
> logical_end_heap_rewrite, logical_rewrite_heap_tuple,
> logical_begin_heap_rewrite.
>
Yes that's exactly it. When table is rewritten we need to create mapping
for every tuple that was created or removed (ie, insert, update or
delete operation happened on it) since the oldest replication slot xmin
for logical decoding to continue to work on that table after the
rewrite. And pg_repack doesn't create that mapping.
-- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training &
Services
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