Re: [HACKERS] logical decoding of two-phase transactions

On 5 January 2017 at 20:43, Stas Kelvich <s.kelvich@postgrespro.ru> wrote:

> Anyway, I can measure WAL space overhead introduced by the GID’s inside commit records
> to know exactly what will be the cost of such approach.

Sounds like a good idea, especially if you remove any attempt to work
with GIDs for !2PC commits at the same time.

I don't think I care about having access to the GID for the use case I
have in mind, since we'd actually be wanting to hijack a normal COMMIT
and internally transform it to PREPARE TRANSACTION, <do stuff>, COMMIT
PREPARED. But for the more general case of logical decoding of 2PC I
can see the utility of having the xact identifier.

If we presume we're only interested in logically decoding 2PC xacts
that are not yet COMMIT PREPAREd, can we not avoid the WAL overhead of
writing the GID by looking it up in our shmem state at decoding-time
for PREPARE TRANSACTION? If we can't find the prepared transaction in
TwoPhaseState we know to expect a following ROLLBACK PREPARED or
COMMIT PREPARED, so we shouldn't decode it at the PREPARE TRANSACTION
stage.

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