Re: pgsql: Support parallel btree index builds.

On 2018-Feb-02, Robert Haas wrote:

> Support parallel btree index builds.

While looking at a complaint related to progress report of parallel
index builds[1], I noticed this comment

+ /*
+ * Execute this worker's part of the sort.
+ *
+ * Unlike leader and serial cases, we cannot avoid calling
+ * tuplesort_performsort() for spool2 if it ends up containing no dead
+ * tuples (this is disallowed for workers by tuplesort).
+ */
+ tuplesort_performsort(btspool->sortstate);
+ if (btspool2)
+ tuplesort_performsort(btspool2->sortstate);

I've been trying to understand why this says "Unlike leader and serial
cases, ...". I understand the "serial" part -- it refers to
_bt_leafbuild. So I'm to understand that that one works differently;
see below. But why does it say "the leader case"? As far as I can see,
the leader executes exactly the same code, so what is the comment
talking about?

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.

[1] https://postgr.es/m/CAEze2Wgm-NnZe3rOnwjYTVriS8xsVhzzVBCGj34h06cDNuaTig@mail.gmail.com

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