Re: Refactoring postgres_fdw/connection.c

On Thu, Jul 28, 2022 at 11:56 AM Fujii Masao
<masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>
>
>
> On 2022/07/27 10:36, Kyotaro Horiguchi wrote:
> > At Tue, 26 Jul 2022 18:33:04 +0900, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote in
> >>> I'm not sure the two are similar with each other. The new function
> >>> pgfdw_exec_pre_commit() looks like a merger of two isolated code paths
> >>> intended to share a seven-line codelet. I feel the code gets a bit
> >>> harder to understand after the change. I mildly oppose to this part.
> >>
> >> If so, we can pgfdw_exec_pre_commit() into two, one is the common
> >> function that sends or executes the command (i.e., calls
> >> do_sql_command_begin() or do_sql_command()), and another is
> >> the function only for toplevel. The latter function calls
> >> the common function and then executes DEALLOCATE ALL things.
> >>
> >> But this is not the way that other functions like
> >> pgfdw_abort_cleanup()
> >> is implemented. Those functions have both codes for toplevel and
> >> !toplevel (i.e., subxact), and run the processings depending
> >> on the argument "toplevel". So I'm thinking that
> >> pgfdw_exec_pre_commit() implemented in the same way is better.
> >
> > I didn't see it from that viewpoint but I don't think that
> > unconditionally justifies other refactoring. If we merge
> > pgfdw_finish_pre_(sub)?commit_cleanup()s this way, in turn
> > pgfdw_subxact_callback() and pgfdw_xact_callback() are going to be
> > almost identical except event IDs to handle. But I don't think we
> > would want to merge them.
>
> I don't think they are so identical because (as you say) they have to handle different event IDs. So I agree we don't want to merge them.
>
>
> > A concern on 0002 is that it is hiding the subxact-specific steps from
> > the subxact callback. It would look reasonable if it were called from
> > two or more places for each topleve and !toplevel, but actually it has
> > only one caller for each. So I think that pgfdw_exec_pre_commit
> > should not do that and should be renamed to pgfdw_commit_remote() or
> > something. On the other hand pgfdw_finish_pre_commit() hides
> > toplevel-specific steps from the caller so the same argument holds.
>
> So you conclusion is to rename pgfdw_exec_pre_commit() to pgfdw_commit_remote() or something?
>
>
> > Another point that makes me concern about the patch is the new
> > function takes an SQL statement, along with the toplevel flag. I guess
> > the reason is that the command for subxact (RELEASE SAVEPOINT %d)
> > requires the current transaction level. However, the values
> > isobtainable very cheap within the cleanup functions. So I propose to
> > get rid of the parameter "sql" from the two functions.
>
> Yes, that's possible. That is, pgfdw_exec_pre_commit() can construct the query string by executing the following codes, instead of accepting the query as an argument. But one downside of this approach is that the following codes are executed for every remote subtransaction entries. Maybe it's cheap to construct the query string as follows, but I'd like to avoid any unnecessray overhead if possible. So the patch makes the caller, pgfdw_subxact_callback(), construct the query string only once and give it to pgfdw_exec_pre_commit().
>
> curlevel = GetCurrentTransactionNestLevel();
> snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
>

Another possibility I can see is that instead of calling
pgfdw_exec_pre_commit() (similarly pgfdw_abort_cleanup) for every
connection entry, we should call that once from the callback function,
and for that we need to move the hash table loop inside that function.

The structure of the callback function looks a little fuzzy to me
where the same event is checked for every entry of the connection hash
table. Instead of simply move that loop should be inside those
function (e.g. pgfdw_exec_pre_commit and pgfdw_abort_cleanup), and let
called those function called once w.r.t to event and that function
should take care of every entry of the connection hash table. The
benefit is that we would save a few processing cycles that needed to
match events and call the same function for each connection entry.

I tried this refactoring in 0004 patch which is not complete, and
reattaching other patches too to make CFboat happy.

Thoughts? Suggestions?

Regards,
Amul