* Claudio Freire (klaussfre...@gmail.com) wrote: > On Tue, Jan 15, 2013 at 8:19 PM, Bruce Momjian <br...@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
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