Re: let's kill AtSubStart_Notify

Hi Robert,

Generally, a subsystem can avoid needing a callback at subtransaction
> start (or transaction start) by detecting new levels of
> subtransactions at time of use.

Yes I agree with this argument.

> A typical practice is to maintain a
> stack which has entries only for those transaction nesting levels
> where the functionality was used. The attached patch implements this
> method for async.c.

I have reviewed your patch, and it seems correctly implementing the
actions per subtransactions using stack. Atleast I could not find
any flaw with your implementation here.

> I was a little surprised to find that it makes a
> pretty noticeable performance difference when starting and ending
> trivial subtransactions. I used this test case:
>
> \timing
> do $$begin for i in 1 .. 10000000 loop begin null; exception when
> others then null; end; end loop; end;$$;
>

I ran your testcase and on my VM I get numbers like 3593.801 ms
without patch and 3593.801 with the patch, average of 5 runs each.
The runs were quite consistent.

Further make check also passing well.

Regards,
Jeevan Ladhe