On Thu, Mar 28, 2013 at 02:03:42PM -0700, Kevin Grittner wrote:
> kelphet xiong <kelphet@gmail.com> wrote:
>
> > When I use postgres and issue a simple sequential scan for a
> > table inventory using query "select * from inventory;", I can see
> > from "top" that postmaster is using 100% CPU, which limits the
> > query execution time. My question is that, why CPU is the
> > bottleneck here and what is postmaster doing? Is there any way to
> > improve the performance? Thanks!
>
> > explain analyze select * from inventory;
> >
> > Seq Scan on inventory (cost=0.00..180937.00 rows=11745000 width=16) (actual time=0.005..1030.403 rows=11745000
loops=1)
> > Total runtime: 1750.889 ms
>
> So it is reading and returning 11.7 million rows in about 1 second,
> or about 88 nanoseconds (billionths of a second) per row. You
> can't be waiting for a hard drive for many of those reads, or it
> would take a lot longer, so the bottleneck is the CPU pushing the
> data around in RAM. I'm not sure why 100% CPU usage would surprise
> you. Are you wondering why the CPU works on the query straight
> through until it is done, rather than taking a break periodically
> and letting the unfinished work sit there?
>
> --
> Kevin Grittner
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
Alternatively, purchase a faster CPU if CPU is the bottleneck as it
is in this case or partition the work into parallel queuries that can
each use a processor.
Regards,
Ken