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

Hello, I added this as Older Bugs in Open items. (I believe it's legit.)

The latest patch still applies on the HEAD with some offsets.

At Tue, 23 Jan 2018 18:50:00 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote in <20180123(dot)185000(dot)232069302(dot)horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
> At Fri, 19 Jan 2018 18:24:56 +0900, Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote in <20180119092456(dot)GA1169(at)paquier(dot)xyz>
> > 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...
>
> I haven't seen it in detail but it seems meaning that it is done
> by notifying something from the standby to the parimary not
> knowing what is a WAL record on the standby side.
>
> > > (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
>
> I'm not sure what the time travel is, but the requried segments
> seems kept safely on logical replication since the publisher
> moves restart_lsn not based on finished commits LSN, but on the
> beginning LSN of running transactions. In other words, logical
> decoding doesn't need to track each record for the purpose of
> keeping segments since it already keeping track of the beginning
> of a transaction. Physical replication is totally unaware of a
> WAL record, it just copies blobs in a convenient unit and only
> cares LSN. But the ignorance seems required to keep performance.
>
> > 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.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center