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

Hi Andres,

>> > what I'm proposing is that that various catalog access functions throw a
>> > new class of error, something like "decoding aborted transactions".
>>
>> When will this error be thrown by the catalog functions? How will it
>> determine that it needs to throw this error?
>
> The error check would have to happen at the end of most systable_*
> functions. They'd simply do something like
>
> if (decoding_in_progress_xact && TransactionIdDidAbort(xid_of_aborted))
> ereport(ERROR, (errcode(DECODING_ABORTED_XACT), errmsg("oops")));
>
> i.e. check whether the transaction to be decoded still is in
> progress. As that would happen before any potentially wrong result can
> be returned (as the check happens at the tail end of systable_*),
> there's no issue with wrong state in the syscache etc.
>

Oh, ok. The systable_* functions use the passed in snapshot and return
tuples matching to it. They do not typically have access to the
current XID being worked upon..

We can find out if the snapshot is a logical decoding one by virtue of
its "satisfies" function pointing to HeapTupleSatisfiesHistoricMVCC.

>
>> The catalog scan will NOT error out but will return metadata which
>> causes the insert-decoding change apply callback to error out.
>
> Why would it not throw an error?
>

In your scheme, it will throw an error, indeed. We'd need to make the
"being-currently-decoded-XID" visible to these systable_* functions
and then this scheme will work.

Regards,
Nikhils

> Greetings,
>
> Andres Freund

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