let's kill AtSubStart_Notify

There are only four subsystems which require a callback at the
beginning of each subtransaction: the relevant functions are
AtSubStart_Memory, AtSubStart_ResourceOwner, AtSubStart_Notify, and
AfterTriggerBeginSubXact. The AtSubStart_Memory and
AtSubStart_ResourceOwner callbacks seem relatively unobjectionable,
because almost every subtransaction is going to allocate memory and
acquire some resource managed by a resource owner, but the others
represent initialization that has to be done whether or not the
corresponding feature is used.

Generally, a subsystem can avoid needing a callback at subtransaction
start (or transaction start) by detecting new levels of
subtransactions at time of use. 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 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 the test four times with and without the patch and took the
median of the last three. This was an attempt to exclude effects due
to starting up the database cluster. With the patch, the result was
3127.377 ms; without the patch, it was 3527.285 ms. That's a big
enough difference that I'm wondering whether I did something wrong
while testing this, so feel free to check my work and tell me whether
I'm all wet. Still, I don't find it wholly unbelievable, because I've
observed in the past that these code paths are lean enough that a few
palloc() calls can make a noticeable difference, and the effect of
this patch is to remove a few palloc() calls.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company