Re: Parallel query execution

* Claudio Freire (klaussfreire@gmail.com) wrote:
> On Tue, Jan 15, 2013 at 8:19 PM, Bruce Momjian <bruce@momjian.us> wrote:
> > The 1GB idea is interesting.  I found in pg_upgrade that file copy would
> > just overwhelm the I/O channel, and that doing multiple copies on the
> > same device had no win, but those were pure I/O operations --- a
> > sequential scan might be enough of a mix of I/O and CPU that parallelism
> > might help.
>
> AFAIR, synchroscans were introduced because multiple large sequential
> scans were counterproductive (big time).

Sequentially scanning the *same* data over and over is certainly
counterprouctive.  Synchroscans fixed that, yes.  That's not what we're
talking about though- we're talking about scanning and processing
independent sets of data using multiple processes.  It's certainly
possible that in some cases that won't be as good, but there will be
quite a few cases where it's much, much better.

Consider a very complicated function running against each row which
makes the CPU the bottleneck instead of the i/o system.  That type of a
query will never run faster than a single CPU in a single-process
environment, regardless of if you have synch-scans or not, while in a
multi-process environment you'll take advantage of the extra CPUs which
are available and use more of the I/O bandwidth that isn't yet
exhausted.
Thanks,
    Stephen