On Mon, Jan 23, 2017 at 12:53 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:

> Corey Huinker <corey.huin...@gmail.com> writes:
> > I was giving some thought to how psql handles undefined variables.
> > I would like an option where either psql can provide an alternate value
> > when an undefined variable is referenced, or a way to detect that a
> > specific variable is undefined and replace it with a defined variable.
> This seems pretty bizarre.  What's the use case?  Why would it not
> be better to build the behavior out of other spare parts, along the
> lines of COALESCE or perhaps
>       \if not defined(x)
>       \set x y
>       \fi
> Obviously the \if stuff is things we don't have yet either, but
> it seems less likely to have surprising side-effects.
>                         regards, tom lane

In light of the backticks variable expansion thread, I'm reviving this
thread in the hopes that a defined()-ish psql function can make it into v10.
It's something that cannot be solved with a query and \gset, so adding it
to psql boolean expressions is the only option I can see.

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