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Subject: Re: logical decoding of two-phase transactions
From: Stas Kelvich <s.kelvich@postgrespro.ru>
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Date: Mon, 20 Mar 2017 15:57:07 +0300
Cc: Petr Jelinek <petr.jelinek@2ndquadrant.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Robert Haas <robertmhaas@gmail.com>
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To: Craig Ringer <craig@2ndquadrant.com>
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> On 20 Mar 2017, at 15:17, Craig Ringer <craig@2ndquadrant.com> wrote:
>=20
>> I thought about having special field (or reusing one of the existing fie=
lds)
>> in snapshot struct to force filtering xmax > snap->xmax or xmin =3D snap=
->xmin
>> as Petr suggested. Then this logic can reside in ReorderBufferCommit().
>> However this is not solving problem with catcache, so I'm looking into i=
t right now.
>=20
> OK, so this is only an issue if we have xacts that change the schema
> of tables and also insert/update/delete to their heaps. Right?
>=20
> So, given that this is CF3 for Pg10, should we take a step back and
> impose the limitation that we can decode 2PC with schema changes or
> data row changes, but not both?

Yep, time is tight. I=E2=80=99ll try today/tomorrow to proceed with this tw=
o scan approach.
If I=E2=80=99ll fail to do that during this time then I=E2=80=99ll just upd=
ate this patch to decode
only non-ddl 2pc transactions as you suggested.

>> Just as before I marking this transaction committed in snapbuilder, but =
after
>> decoding I delete this transaction from xip (which holds committed trans=
actions
>> in case of historic snapshot).
>=20
> That seems kind of hacky TBH. I didn't much like marking it as
> committed then un-committing it.
>=20
> I think it's mostly an interface issue though. I'd rather say
> SnapBuildPushPrepareTransaction and SnapBuildPopPreparedTransaction or
> something, to make it clear what we're doing.

Yes, that will be less confusing. However there is no any kind of queue, so
SnapBuildStartPrepare / SnapBuildFinishPrepare should work too.

> --=20
> Craig Ringer                   http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services


Stas Kelvich
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company


--=20
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