Re: Using quicksort for every external sort run

On Sun, Mar 20, 2016 at 11:01 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> Allowing 0 tuple runs in rare cases seems like the simplest solution.
> After all, mergeprereadone() is expressly prepared for 0 tuple runs.
> It says "ensure that we have at least one tuple, if any are to be
> had". There is no reason to assume that it says this only because it
> imagines that no tuples might be found *only after* the first preread
> for the merge (by which I mean I don't think that only applies when a
> final on-the-fly merge reloads tuples from one particular tape
> following running out of tuples of the tape/run in memory).

I just realized that there is what amounts to an over-zealous
assertion in dumpbatch():

> + * When this edge case hasn't occurred, the first memtuple should not
> + * be found to be heapified (nor should any other memtuple).
> + */
> + Assert(state->memtupcount == 0 ||
> + state->memtuples[0].tupindex == HEAP_RUN_NEXT);

The problem is that state->memtuples[0].tupindex won't have been
*reliably* initialized here. We could make sure that it is for the
benefit of this assertion, but I think it would be better to just
remove the assertion, which isn't testing very much over and above the
similar assertions that appears in the only dumpbatch() caller,
dumptuples().

--
Peter Geoghegan