Re: Eliminating CREATE INDEX comparator TID tie-breaker overhead

On Tue, Jul 21, 2015 at 4:06 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> Design considerations and consequences
> --------------------------------------------------------

Good write-up.

> I'm not concerned about synchronized scans breaking my assumption of a
> physical ordering of heaptuples being fed to tuplesort.c. I think that
> it is unlikely to ever be worth seriously considering this case.

Why not?

> I have a hard time imagining anything (beyond synchronous scans)
> breaking my assumption that index tuplesorts receive tuples in heap
> physical order. If anything was to break that in the future (e.g.
> parallelizing the heap scan for index builds), then IMV the onus
> should be on that new case to take appropriate precautions against
> breaking my assumption.

I'm very dubious about that. There are lots of reasons why we might
want to read tuples out of order; for example, suppose we want a
parallel sequential scan to feed the sort.

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