Re: [HACKERS] [BUGS] Bug in Physical Replication Slots (at least 9.5)?

From: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: andres(at)anarazel(dot)de, sfrost(at)snowman(dot)net, nag1010(at)gmail(dot)com, jdnelson(at)dyn(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] [BUGS] Bug in Physical Replication Slots (at least 9.5)?
Date: 2018-01-19 09:24:56
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On Fri, Jan 19, 2018 at 10:54:53AM +0900, Kyotaro HORIGUCHI wrote:
> On the other hand if one logical record must be read from single
> source, we need any means to deterrent wal-recycling on the
> primary side. Conducting that within the primary side is rejected
> by consensus.

There is this approach of extending the message protocol as well so as
the primary can retain the segments it needs to keep around...

> (There might be smarter way to do that, though.) To
> do that from the standby side, just retarding write feedbacks or
> this kind of special feedback would be needed. In any way it is
> necessary to introduce WAL-record awareness into WAL shipping
> process and it's seemingly large complexity.

Coming to logical slots, don't you need to be aware of the beginning of
the record on the primary to perform correctly decoding of tuples
through time travel? If the record is present across segments, it seems
to me that it matters. Andres knows the answer to that for sure, so I
would expect to be counter-argued in the next 12 hours or so.

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