Re: count(*) performance improvement ideas

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Magnus Hagander <magnus(at)hagander(dot)net>, Stephen Denne <Stephen(dot)Denne(at)datamail(dot)co(dot)nz>, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, Mark Mielke <mark(at)mark(dot)mielke(dot)cc>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: count(*) performance improvement ideas
Date: 2008-06-23 22:12:20
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Added to TODO:

* Allow custom variables to appear in pg_settings()


Tom Lane wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> > Tom Lane wrote:
> >> Really? [ pokes around ... ] Hm, you're right, because
> >> add_placeholder_variable() sets the GUC_NO_SHOW_ALL flag, and in this
> >> usage it'll never be cleared. I wonder if we should change that.
> >>
> >> The whole thing is a bit of an abuse of what the mechanism was
> >> intended for, and so I'm not sure we should rejigger GUC's behavior
> >> to make it more pleasant, but on the other hand if we're not ready to
> >> provide a better substitute ...
> > While I agree with that part, is there any actual *reason* why we
> > shouldn't have the custom variables included in pg_settings?
> IIRC, the motivation for doing that was to not expose a completely bogus
> set of attributes for a variable whose defining C-module hadn't been
> loaded yet.
> I thought about this in the shower just now, and ISTM that if we want to
> turn this into an actual feature rather than a kluge, there needs to be
> some sort of "define variable" command that sets up a custom variable
> and specifies its type (and whatever other properties seem worth
> setting). IOW expose the DefineCustomFooVariable functions to SQL users.
> I'd be a bit inclined to restrict the namespace that can be set up that
> way, eg allow only "local." or "session." as the prefix. Maybe
> that's just being too anal, but we could guarantee not to introduce
> colliding built-in GUCs in future releases, whereas people trying to
> define variables with any random name would definitely be at risk.
> Comments?
> regards, tom lane
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Bruce Momjian <bruce(at)momjian(dot)us>

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