From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, Álvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Rahila Syed <rahilasyed90(at)gmail(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Masao Fujii <masao(dot)fujii(at)gmail(dot)com>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, pokurev(at)pm(dot)nttdata(dot)co(dot)jp, Vinayak Pokale <vinpokale(at)gmail(dot)com> |
Subject: | Re: [PROPOSAL] VACUUM Progress Checker. |
Date: | 2015-12-10 14:49:12 |
Message-ID: | 12757.1449758952@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Oh, please, no. Gosh, this is supposed to be a lightweight facility!
> Just have a chunk of shared memory and write the data in there. If
> you try to feed this through the stats collector you're going to
> increase the overhead by 100x or more, and there's no benefit. We've
> got to do relation stats that way because there's no a priori bound on
> the number of relations, so we can't just preallocate enough shared
> memory for all of them. But there's no similar restriction here: the
> number of backends IS fixed at startup time. As long as we limit the
> amount of progress information that a backend can supply to some fixed
> length, which IMHO we definitely should, there's no need to add the
> expense of funneling this through the stats collector.
I agree with this, and I'd further add that if we don't have a
fixed-length progress state, we've overdesigned the facility entirely.
People won't be able to make sense of anything that acts much more
complicated than "0% .. 100% done". So you need to find a way of
approximating progress of a given command in terms more or less
like that, even if it's a pretty crude approximation.
regards, tom lane
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