| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
| Cc: | Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>, Robert Haas <robertmhaas(at)gmail(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Fabrízio Mello <fabriziomello(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Seq Scan |
| Date: | 2015-07-07 06:33:09 |
| Message-ID: | 1436250789.4369.160.camel@jeff-desktop |
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| Lists: | pgsql-hackers |
On Tue, 2015-07-07 at 09:27 +0530, Amit Kapila wrote:
> I am not sure how many blocks difference could be considered okay for
> deviation?
In my testing (a long time ago) deviations of tens of blocks didn't show
a problem.
However, an assumption of the sync scan work was that the CPU is
processing faster than the IO system; whereas the parallel scan patch
assumes that the IO system is faster than a single core. So perhaps the
features are incompatible after all. Only testing will say for sure.
Then again, syncscans are designed in such a way that they are unlikely
to hurt in any situation. Even if the scans diverge (or never converge
in the first place), it shouldn't be worse than starting at block zero
every time.
I'd prefer to leave syncscans intact for parallel scans unless you find
a reasonable situation where they perform worse. This shouldn't add any
complexity to the patch (if it does, let me know).
Regards,
Jeff Davis
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