Re: Variable substitution in psql backtick expansion

On Fri, Mar 31, 2017 at 2:33 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Awhile back in the discussion about the \if feature for psql,
> I'd pointed out that you shouldn't really need very much in
> the way of boolean-expression evaluation smarts, because you
> ought to be able to use a backtick shell escape:
>
>         \if `expr :foo \> :bar`
>                 \echo :foo is greater than :bar
>         \endif
>
> Now that the basic feature is in, I went to play around with this,
> and was disappointed to find out that it doesn't work.  The reason
> is not far to seek: we do not do variable substitution within the
> text between backticks.  psqlscanslash.l already foresaw that some
> day we'd want to do that:
>
>         /*
>          * backticked text: copy everything until next backquote, then evaluate.
>          *
>          * XXX Possible future behavioral change: substitute for :VARIABLE?
>          */
>
> I think today is that day, because it's going to make a material
> difference to the usability of this feature.
>
> I propose extending backtick processing so that
>
> 1. :VARIABLE is replaced by the contents of the variable
>
> 2. :'VARIABLE' is replaced by the contents of the variable,
> single-quoted according to Unix shell conventions.  (So the
> processing would be a bit different from what it is for the
> same notation in SQL contexts.)
>
> This doesn't look like it would take very much new code to do.
>
> Thoughts?

In short, +1.
-- 
Michael