Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

On Fri, May 15, 2020 at 4:35 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, May 15, 2020 at 4:20 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Fri, May 15, 2020 at 4:04 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> >
> > >
> > > > - In the example we can not show a real example, because of the
> > > > in-progress transaction to show the changes, we might have to
> > > > implement a lot of tuple. I think we can show the partial output?
> > > >
> > >
> > > I think we can display what API will actually display, what is the
> > > confusion here.
> >
> > What, I meant is that even with the logical_decoding_work_mem=64kb, we
> > need to have quite a few changes in a transaction to stream it so the
> > example output will be quite big in size. So I told we might not show
> > the real example instead we will just show a few lines and cut the
> > remaining. But, I got your point we can just show how it will look
> > like.
> >
>
> Right.
>
> > >
> > > I have a few more comments on the previous version of patch
> > > v20-0005-Implement-streaming-mode-in-ReorderBuffer. If you have fixed
> > > any, then leave those and fix others.
> > >
> > > Review comments:
> > > ------------------------------
> > > 1.
> > > @@ -1762,10 +1952,16 @@ ReorderBufferCommit(ReorderBuffer *rb,
> > > TransactionId xid,
> > > }
> > >
> > > case REORDER_BUFFER_CHANGE_MESSAGE:
> > > - rb->message(rb, txn, change->lsn, true,
> > > - change->data.msg.prefix,
> > > - change->data.msg.message_size,
> > > - change->data.msg.message);
> > > + if (streaming)
> > > + rb->stream_message(rb, txn, change->lsn, true,
> > > + change->data.msg.prefix,
> > > + change->data.msg.message_size,
> > > + change->data.msg.message);
> > > + else
> > > + rb->message(rb, txn, change->lsn, true,
> > > + change->data.msg.prefix,
> > > + change->data.msg.message_size,
> > > + change->data.msg.message);
> > >
> > > Don't we need to set any_data_sent flag while streaming messages as we
> > > do for other types of changes?
> >
> > Actually, pgoutput plugin don't send any data on stream_message. But,
> > I agree that how other plugin will handle. I will analyze this part
> > again, maybe we have to such flag at the plugin level and whether stop
> > is sent to not can also be handled at the plugin level.
> >
>
> Okay, lets discuss this after your analysis.
>
> > > 2.
> > > + if (streaming)
> > > + {
> > > + /*
> > > + * Set the last of the stream as the final lsn before calling
> > > + * stream stop.
> > > + */
> > > + if (!XLogRecPtrIsInvalid(prev_lsn))
> > > + txn->final_lsn = prev_lsn;
> > > + rb->stream_stop(rb, txn);
> > > + }
> > >
> > > I am not sure if it is good to use final_lsn for this purpose. See
> > > comments for this variable in reorderbuffer.h. Basically, it is used
> > > for a specific purpose on different occasions. Now, if we want to
> > > start using it for a new purpose, we need to study its interaction
> > > with all other places and update the comments as well. Can we pass an
> > > additional parameter to stream_stop() instead?
> >
> > I think it was in sycn with the spill code right? I mean the last
> > change we spill is set as the final_lsn and same is done here.
> >
>
> But we use final_lsn in ReorderBufferRestoreCleanup() for serialized
> changes. Now, in some case if we first do serialization, then perform
> streaming and then tried to call ReorderBufferRestoreCleanup(),it
> might not work as intended. Now, this might not happen today but I
> don't think we have any protection to avoid that.

If streaming is complete then we will remove the serialize flag so it
will not cause any issue. However, we can avoid setting final_lsn
here and pass some parameters to the stream_stop about the last lsn of
the stream.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com