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

On Tue, Jun 2, 2020 at 7:53 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Tue, Jun 2, 2020 at 4:56 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Tue, Jun 2, 2020 at 3:59 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > > I thin for our use case BufFileCreateShared is more suitable. I think
> > > we need to do some modifications so that we can use these apps without
> > > SharedFileSet. Otherwise, we need to unnecessarily need to create
> > > SharedFileSet for each transaction and also need to maintain it in xid
> > > array or xid hash until transaction commit/abort. So I suggest
> > > following modifications in shared files set so that we can
> > > conveniently use it.
> > > 1. ChooseTablespace(const SharedFileSet fileset, const char name)
> > > if fileset is NULL then select the DEFAULTTABLESPACEOID
> > > 2. SharedFileSetPath(char path, SharedFileSet fileset, Oid tablespace)
> > > If fileset is NULL then in directory path we can use MyProcPID or
> > > something instead of fileset->creator_pid.
> > >
> >
> > Hmm, I find these modifications a bit ad-hoc. So, not sure if it is
> > better than the patch maintains sharedfileset information.
>
> I think we might do something better here, maybe by supplying function
> pointer or so, but maintaining sharedfileset which contains different
> tablespace/mutext which we don't need at all for our purpose also
> doesn't sound very appealing.
>

I think we can say something similar for Relation (rel cache entry as
well) maintained in LogicalRepRelMapEntry. I think we only need a
pointer to that information.

> Let me see if I can not come up with
> some clean way of avoiding the need to shared-fileset then maybe we
> can go with the shared fileset idea.
>

Fair enough.
..

> > >
> > > But, even if nsubxacts become 0 we want to write the file so that we
> > > can overwrite the previous info.
> > >
> >
> > Can't we just remove the file for such a case?
>
> But, as of now, we expect if it is not a first-time stream start then
> the file exists.
>

Isn't it primarily because we do subxact_info_write in stop stream
which will create such a file irrespective of whether we have any
subxacts? If so, isn't that an unnecessary write?

> Actually, currently, it's very easy that if it is
> not the first segment we always expect that the file must exist,
> otherwise an error.
>

I think we can check if the file doesn't exist then we can initialize
nsubxacts as 0.

> Now if it is not the first segment then we will
> need to handle multiple cases.
>
> a) subxact_info_read need to handle the error case, because the file
> may not exist because there was no subxact in last stream or it was
> deleted because nsubxact become 0.
> b) subxact_info_write, there will be multiple cases that if nsubxact
> was already 0 then we can avoid writing the file, but if it become 0
> now we need to remove the file.
>
> Let me think more on that.
>

I feel we should be able to deal with these cases but if you find any
difficulty then let us discuss. I understand there is some ease if we
always have subxacts file but OTOH it sounds quite awkward that we
need so many file operations to detect the case whether the
transaction has any subtransactions.

> >
> > Here, we have performed Rolledback to savepoint s1 which doesn't have
> > any change of its own. I think this would have handled but just
> > wanted to confirm.
>
> But internally, that will send abort for the s2 first, and for that,
> we will find xid and truncate, and later we will send abort for s1 but
> that we will not find and do nothing? Anyway, I will test it and let
> you know.
>

It would be good if we can test and confirm this behavior once. If it
is not very inconvenient then we can even try to include a test for
the same in the patch.

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