Re: pgsql: Support parallel btree index builds.

On Mon, Jun 7, 2021 at 4:11 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> Now, if you do look at _bt_leafbuild(), it can be seen that nothing is
> done differently there either; we're not actually skipping any calls to
> tuplesort_performsort(). Any differentiation between serial/leader/
> worker cases seems to be done inside that routine. So the comment is
> not very useful there either.
>
> I am wondering if these comments are leftovers from early development
> versions of this patch. Maybe we could remove them -- or rewrite them
> to indicate not that we avoid calling tuplesort_performsort(), but
> instead to say that that function behaves differently.

It's talking about something described in the tuplesort.h contract. It
applies to a tuplesort state, not a process -- the leader always has two
tuplesort states (the leader tuplesort state, plus its own worker
tuplesort state).

The leader tuplesort is very much like a serial tuplesort. In
particular, as step 8 in tuplesort.h points out, the leader doesn't
have to call tuplesort_performsort() for the leader tuplesort state if
it already knows that there is no input to sort.

This matters less than it might in a world where we had a user of
parallel tuplesort that doesn't always simply make the leader
participate as a worker. There is a build-time testing option in
nbtsort.c that does this for parallel CREATE INDEX, actually -- see
DISABLE_LEADER_PARTICIPATION.

You kind of have a point about this being something that made more
sense in revisions of the patch from before commit, though. There was
a question about the cost model and the role of the leader that was
ultimately resolved by inventing the current simple behavior. So
feel free to change the wording now.

--
Peter Geoghegan