On Tue, Nov 11, 2014 at 9:42 AM, Haribabu Kommi <kommi.harib...@gmail.com> wrote: > > On Tue, Nov 11, 2014 at 2:35 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Tue, Nov 11, 2014 at 5:30 AM, Haribabu Kommi < kommi.harib...@gmail.com> > > wrote: > >> > >> On Tue, Nov 11, 2014 at 10:21 AM, Andres Freund <and...@2ndquadrant.com > > >> wrote: > >> > On 2014-11-10 10:57:16 -0500, Robert Haas wrote: > >> >> Does parallelism help at all? > >> > > >> > I'm pretty damn sure. We can't even make a mildly powerfull storage > >> > fully busy right now. Heck, I can't make my workstation's storage with a > >> > raid 10 out of four spinning disks fully busy. > >> > > >> > I think some of that benefit also could be reaped by being better at > >> > hinting the OS... > >> > >> Yes, it definitely helps but not only limited to IO bound operations. > >> It gives a good gain for the queries having CPU intensive where > >> conditions. > >> > >> One more point we may need to consider, is there any overhead in passing > >> the data row from workers to backend? > > > > I am not sure if that overhead will be too much visible if we improve the > > use of I/O subsystem by making parallel tasks working on it. > > I feel there may be an overhead because of workers needs to put the result > data in the shared memory and the backend has to read from there to process > it further. If the cost of transfering data from worker to backend is more than > fetching a tuple from the scan, then the overhead is visible when the > selectivity is more. > > > However > > another idea here could be that instead of passing tuple data, we just > > pass tuple id, but in that case we have to retain the pin on the buffer > > that contains tuple untill master backend reads from it that might have > > it's own kind of problems. > > Transfering tuple id doesn't solve the scenarios if the node needs any > projection.
Hmm, that's why I told that we need to retain buffer pin, so that we can get the tuple data. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
