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

On 29/03/18 23:58, Andres Freund wrote:
> On 2018-03-29 23:52:18 +0200, Tomas Vondra wrote:
>>> I have added details about this in src/backend/storage/lmgr/README as
>>> suggested by you.
>>>
>>
>> Thanks. I think the README is a good start, but I think we also need to
>> improve the comments, which is usually more detailed than the README.
>> For example, it's not quite acceptable that LogicalLockTransaction and
>> LogicalUnlockTransaction have about no comments, especially when it's
>> meant to be public API for decoding plugins.
>
> FWIW, for me that's ground to not accept the feature. Burdening output
> plugins with this will make their development painful (because they'll
> have to adapt regularly) and correctness doubful (there's nothing
> checking for the lock being skipped). Another way needs to be found.
>

I have to agree with Andres here. It's also visible in the latter
patches. The pgoutput patch forgets to call these new APIs completely.
The test_decoding calls them, but it does so even when it's processing
changes for committed transaction.. I think that should be avoided as it
means potentially doing SLRU lookup for every change. So doing it right
is indeed not easy.

I as wondering how to hide this. Best idea I had so far would be to put
it in heap_beginscan (and index_beginscan given that catalog scans use
is as well) behind some condition. That would also improve performance
because locking would not need to happen for syscache hits. The problem
is however how to inform the heap_beginscan about the fact that we are
in 2PC decoding. We definitely don't want to change all the scan apis
for this. I wonder if we could add some kind of property to Snapshot
which would indicate this fact - logical decoding is using it's own
snapshots it could inject the information about being inside the 2PC
decoding.

--
Petr Jelinek http://www.2ndQuadrant.com/
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