Re: Undefined psql variables

From: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Undefined psql variables
Date: 2017-04-02 15:58:00
Message-ID: CADkLM=eVx+qOrcFG-ZkPmneMDR+xC9tBjxbkz9oxVW0gmh2s2w@mail.gmail.com
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On Mon, Jan 23, 2017 at 12:53 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Corey Huinker <corey(dot)huinker(at)gmail(dot)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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