Re: PATCH: logical_work_mem and logical streaming of largein-progress transactions - Mailing list pgsql-hackers

On Thu, Jan 30, 2020 at 6:10 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Thu, Jan 30, 2020 at 4:11 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > Also, if we need to copy the snapshot here, then do we need to again
> > copy it in ReorderBufferProcessTXN(in below code and in catch block in
> > the same function).
> I think so because as part of the
> "REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT" change, we might directly
> point to the snapshot and that will get truncated when we truncate all
> the changes of the ReorderBufferTXN.   So I think we can check if
> snapshot_now->copied is true then we can avoid copying otherwise we
> can copy?
>

Yeah, that makes sense, but I think then we also need to ensure that
ReorderBufferStreamTXN frees the snapshot only when it is copied.  It
seems to me it should be always copied in the place where we are
trying to free it, so probably we should have an Assert there.

One more thing:
ReorderBufferProcessTXN()
{
..
+ if (streaming)
+ {
+ /*
+ * While streaming an in-progress transaction there is a
+ * possibility that the (sub)transaction might get aborted
+ * concurrently.  In such case if the (sub)transaction has
+ * catalog update then we might decode the tuple using wrong
+ * catalog version.  So for detecting the concurrent abort we
+ * set CheckXidAlive to the current (sub)transaction's xid for
+ * which this change belongs to.  And, during catalog scan we
+ * can check the status of the xid and if it is aborted we will
+ * report an specific error which we can ignore.  We might have
+ * already streamed some of the changes for the aborted
+ * (sub)transaction, but that is fine because when we decode the
+ * abort we will stream abort message to truncate the changes in
+ * the subscriber.
+ */
+ CheckXidAlive = change->txn->xid;
+ }
..
}

I think it is better to move the above code into an inline function
(something like SetXidAlive).  It will make the code in function
ReorderBufferProcessTXN look cleaner and easier to understand.

> Other comments look fine to me so I will reply to them along with the
> next version of the patch.
>

Okay, thanks.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com