Re: Parallel Seq Scan

On Wed, Jan 28, 2015 at 8:27 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> I feel like one of us is misunderstanding the numbers, which is probably
> in part because they're a bit piecemeal over email, but the seqscan
> speed in this case looks pretty close to dd performance for this
> particular test, when things are uncached. Cached numbers are
> different, but that's not what we're discussing here, I don't think.
>
> Don't get me wrong- I've definitely seen cases where we're CPU bound
> because of complex filters, etc, but that doesn't seem to be the case
> here.

To try to clarify a bit: What we've testing here is a function I wrote
called parallel_count(regclass), which counts all the visible tuples
in a named relation. That runs as fast as dd, and giving it extra
workers or prefetching or the ability to read the relation with
different I/O patterns never seems to speed anything up very much.

The story with parallel sequential scan itself may well be different,
since that has a lot more CPU overhead than a dumb-simple
tuple-counter.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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