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

On Tue, Jan 28, 2020 at 1:30 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Jan 28, 2020 at 11:58 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Tue, Jan 28, 2020 at 11:43 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > > > It seems to me that we need to add all of this new handling because
> > > > > while taking the decision whether to stream or not we don't know
> > > > > whether the txn has changes that can't be streamed. One idea to make
> > > > > it work is that we identify it while decoding the WAL. I think we
> > > > > need to set a bit in the insert/delete WAL record to identify if the
> > > > > tuple belongs to a toast relation. This won't add any additional
> > > > > overhead in WAL and reduce a lot of complexity in the logical decoding
> > > > > and also decoding will be efficient. If this is feasible, then we can
> > > > > do the same for speculative insertions.
> > > > The Idea looks good to me. I will work on this.
> > > >
> > >
> > > One more thing we can do is to identify whether the tuple belongs to
> > > toast relation while decoding it. However, I think to do that we need
> > > to have access to relcache at that time and that might add some
> > > overhead as we need to do that for each tuple. Can we investigate
> > > what it will take to do that and if it is better than setting a bit
> > > during WAL logging.
> >
> > IMHO, for the catalog scan, we will have to start/stop the transaction
> > for each change. So do you want that we should evaluate its
> > performance?
> >
>
> No, I was not thinking about each change, but at the level of ReorderBufferTXN.
That means we will have to keep that transaction open until we decode
the commit WAL for that ReorderBufferTXN or you have anything else in
mind?

>
> > Also, during we get the change we might not have the
> > complete historic snapshot ready to fetch the rel cache entry.
> >
>
> Before decoding each change (say DecodeInsert), we call
> SnapBuildProcessChange. Isn't that sufficient?
Yeah, Right, we can get some recache entry based on the base snapshot.
And, that might be sufficient to know whether it's a toast relation or
not.
>
> Even, if the above is possible, I am not sure how good is it for each
> change we fetch rel cache entry, that is the point I was worried.

We might not need to scan the catalog every time, we might get it from
the cache itself.

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