Re: Variable substitution in psql backtick expansion

"Daniel Verite" <daniel(at)manitou-mail(dot)org> writes:
> ISTM that expr is too painful to use to be seen as the
> idiomatic way of achieving comparison in psql.

I'm not proposing it as the best permanent solution, just saying
that having this in v10 is a lot better than having nothing in v10.
And we certainly aren't going to get any more-capable boolean
expression syntax into v10 at this point.

> Among its disadvantages, it won't work on windows, and its
> interface is hard to work with due to the necessary
> quoting of half its operators, and the mandatory spacing
> between arguments.
> Also the quoting rules and command line syntax
> depend on the underlying shell.

All true, but that's true of just about any use of backtick
or \! commands. Shall we remove those features because they
are hard to use 100% portably?

> Isn't it going to be tricky to produce code that works
> across different families of shells, like bourne and csh?

Probably 95% of our users do not really care about that,
because they're only trying to produce scripts that will
work in their own environment.

> I think that users would rather have the option to just put
> an SQL expression behind \if. That implies a working connection
> to evaluate, which expr doesn't, but that's no
> different from the other backslash commands that read
> the database.

Well, it also implies being in a non-aborted transaction,
which seems like more of a problem to me for \if scripting.
Certainly there would be value in an option like that, but
it's not any more of a 100% solution than the `expr` one is.

Also, I didn't think I'd have to point it out, but expr(1)
is hardly the only command you can call from a backtick
substitution. There are *lots* of potential use-cases
here ... but not so many if you can't plug any variable
material into the shell command.

regards, tom lane