Re: Reducing overhead for repeat de-TOASTing

On Wed, 2008-06-18 at 09:45 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > You've not covered the idea that we just alter the execution so we just
> > detoast once.
>
> That's because I already considered and rejected that idea. There's
> no very good place to do it. See thread on postgis-devel:
>
> http://postgis.refractions.net/pipermail/postgis-devel/2008-June/003091.html
>
> Aside from the problems mentioned there, there's the issue that a lower
> plan level doesn't have any way to know whether the value will be needed
> at all. We could look for references to the Var but it's entirely
> possible that the Var is being passed to some function that doesn't
> require a fully detoasted result. It wouldn't do for this
> "optimization" to disable the slice-fetch feature...

Agreed. Yet I'm thinking that a more coherent approach to optimising the
tuple memory usage in the executor tree might be better than the special
cases we seem to have in various places. I don't know what that is, or
even if its possible though.

--
Simon Riggs www.2ndQuadrant.com
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