Re: count(*) performance improvement ideas

Added to TODO:
* Allow custom variables to appear in pg_settings()


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Tom Lane wrote:
> Magnus Hagander <magnus@hagander.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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